region of the galaxy, very similar to the filaments inthat other famous southern hot-spot galaxy NGG 1097(Wolstencroft and Zealey 1975).

Summary

The experience with the 4 cm and 9 cm cameras incombination with the 1.5 m telescope has been extremelysatisfactory. The power of the large camera is illustrated byfigure 3 which should be compared with the 30-minuteelectronograph of the same area shown in MessengerNo. 16, p. 1. The IIford emulsions used from the beginningof the test period have turned out to be of good quality andwith very few defects occurring, so that practically all thefilms provided could be used.

With the good seeing experienced in the 1.5 m telescopedome and the very fine external seeing occurring occa­sionally, coupled with the high quality of the telescopeoptics (Andersen and Niss 1979), the electronographicresolution mentioned above will permit a very faint limitingmagnitude with this combination of instruments. Limits ofabout 26 magnitude for stars and 27 'magnitude persquare arc second for extended objects should be ob­tained under optimum weather conditions.

The collaboration described has thus been successful. Itshould be added that the achievements during the periodin question have also, at least partly, encouraged theproduction of similar electronographic equipment forother non-British telescopes such as the ESO 3.6 mtelescope on La Silla.

Acknowledgements

The authors thank staff members at RGO and at GUO for alltheir assistance. The Danish Natural Science ResearchGouncil awarded a special grant for the Danish part of theproject and financed Finn Johannessen's work at RGO.A NATO Research Grant has provided essential support forthe Danish-British collaboration.

References

Andersen, J., FIorenIin, R. and Gyldenkerne, K. 1979 The Mes­senger, 16, 1.

Andersen, J. and Niss, B. 1979, The Messenger, 17, p. 14.

Discovery of a New EclipsingDwarf Nova: OY CarinaeThe weather at the beginning of the night April 29/30, 1979was not excellent on La Silla: Most clouds were gone, butmany of them were just waiting near the horizon andthreatened to come back. After twilight I pointed the ESO1 m telescope to the southern dwarf nova OY Gar, which isa faint variable star of about 16~. The first photoelectricmeasu rement, however, revealed 14~'8, a bit brighter thannormal, and it seemed to brighten up rapidly. Of course,this is not unusual for a dwarf nova: OY Gar just wasbeginning one of its eruptions. lieft the telescope on thestar monitoring it continuously in 3-second time intervals.

Fig. 3: A 90-minute exposure of a region around NGC 2081 in theLMC, taken with the 9 cm McMullan camera on the Oanish 1.5 mtelescope. Emulsion lIford G 5 and filter "Oark Sky Blue". Theexposure may be compared with the one shown on page 1 ofMessenger No. 16.

BoiIon, J. G. and Savage, A. 1978, lAU Symposium No. 79, 295.Chincarini, G. L., Griboval, P.J. and Smilh, H.J. (ed.) 1974,

Proceedings, Conlerence on Eleclrography and AslronomicalApplications, Universily 01 Texas.

Disney, M. J. 1976, Nature, 263, 573.Gyldenkerne, K., Axon, D. and Taylor, K. 1979, in preparalion.McMullan, 0., Powell, J. R. and Curlis, N. A., 1972, Adv. EEP, Vol.

33A, p. 37.McMullan, 0., Powell, J. R. and Curtis, N. A., 1976, Adv. EEP,

Vol. 40 B, p. 627.McMullan, D. and Powell, J. R. 1976, Proceedings, lAU Colloquium

No. 40,5.McMullan, D. and Powell, J. R. 1979, Adv. EEP, in press.Wolstencrofl, R. D. and Zealey, W. J. 1975, Mon. Not. R. Astr. Soc.,

173,91 p.

After five minutes, however, a new surprise showed up: theintensity began to drop! I checked the diaphragm: the starwas properly centred. I looked at the sky: No visible cloudsin the field. But the signal diminished more and more, andwithin a few minutes the star was five times fainter thanbefore! This did not last long: the intensity began to riseagain, even faster than the decline, and reached itsprevious value five minutes later. Gould this have beencaused by the unstable weather conditions? I kept thetelescope on OY Gar and found out that the darkening ofthis star repeated periodically every 91 minutes: no cloudis known to pass so regularly!

There was no doubt that I had discovered a new ecl ipsi ngdwarf nova with an extremely short period. Figure 1 showsthe first eclipse ever observed: the star was still faint (-15m

)

at this time. Two totality phases appear like "steps" in thelightcurve. According to recent dwarf nova models, thefirst "step" corresponds to the total eclipse of the centralparts of the disk and of the white dwarf, while the second

39

an o'utburst and-Iast but not least­an excellent cooperation of HolgerPedersen who was scheduled for halfof the observing nights, but kindly leftme the critical hours for OY Gar.

Nikolaus Vogt

Fig. 1: Eclipse lightcurves of OY Gar,observed on April 29/30, 1979 (uppercurve)and April30/May 1 (Iower curve) with theESO 1 m telescope. The uppercurveshowstwo totality phases which are indicated bybars (explanation see text). Each curvecovers a total time interval of about 13 mi­nutes.

OY Car

;,':'/.",,':'

TIME

: '.".

.........'.'

",-.

.'~

....'..

una magnitud de s610 0.006! Esto expliea elporque hasta ahora la variabilidad de laestrella no habfa sido deteetada antes,a pesar de que esta estrella habia sidoobservada muehas veees.

EI deseubrimiento es una demostraei6nde la exeelente ubieaei6n de La Silla y deibuen rendimiento dei teleseopio de 61 emde Boehum y su fot6metro, eon el eual seefeetuaron las observaeiones.

Nuevas seriesde diapositivas de ESODurante los pr6ximos meses se dispondrade dos nuevas series de diapositivas.

La primera de estas eonsiste de 20 diapo­sitivas de 5 x 5 em que muestran las insta­laeiones en La Silla. Ineluyen edifieios,teleseopios y vistas dei lugar. Una deserip­ei6n eompleta en varios idiomas expliea lasdiapositivas.

La segunda serie eontiene algunas de lasmejores fotogratras que han sido tomadoseon la eamera dei foeo primario dei tele­seopio de 3.6 m (eorreetor Gaseoigne). Oeentre mas de 1.000 fotogratras, se eligieron20 diapositivas en blaneo y negro (nebulo­sas, galaxias, ete.).

EI preeio por una serie de diapositivas esde DM 18.- (0 su equivalente) en Europa,y US$ 10,- por eorreo terrestre a todos losdemas pafses. 0 US$ 12.50 para su enviopor vfa aerea (pagadero por adelantado).

Durante una reeiente estadia en La Silla elDr. Erie W. Eist dei Observatorio Royal enUeele, Belgiea, ha deseubierto que una delas estrellas en la Cruz dei Sur, Ii Crueis, esvariable.

Naturalmente hay muehas otras estrellasvariables, sin embargo. el presente easo espartieularmente interesante porque la am­plitud maxima en la eurva luminosa tiene

ö Crucis es variable!

ALGUNOS RESUMENES

La calidad dei telescopiodanes de 1.5 m sobrepasalas expectaciones.

Desde fines de noviembre dei ario pasadoun nuevo instrumento se eneuentra a dis­posiei6n en La Silla: EI teleseopio danes de1.5 m.

En una serie de plaeas tomadas duranteuna noehe de exeelentes eondieiones at­mosferieas a prineipios de marzo dei pre­sente ario, el tamario de las imagenesvariaba de 1 segundo de areo hasta 0.6 se­gundo de areo. La mejor plaea-eon unaexposiei6n de una hora-muestra bonitasimagenes eireulares de 0.5 segundo deareo! Es este un resultado easi inerefble,que eomprueba la exeelente ealidad deestenuevo teleseopio.

and deepest "step" coincides with thecoverage of the "hot spot". Thesecond lightcurve shown in figure 1corresponds to the next night whenOY Gar had reached the maximumbrightness of its eruption (-12':'4).Only a partial eclipse is observed be­cause the eclipsed body (the disk) isnow much more extended than before,and cannot any more be covered total­Iy by the faint red secondary star.Since we see the hot spot radiationseparated from that of the disk atcertain eclipse phases, we can calcu­late the relative contribution of bothcomponents to the total light, and canfollow up this ratio throughout theoutburst. This will have important con­sequences for the dwarf nova outbu rstmechanism which is still not definitelyknown.

The discovery of this eclipsing bi na­ry was not a pu re accident: in January1979, when I took spectrograms ofseveral dwarf novae with the ImageDissector Scanner at the 3.6 m tele­scope, OY Gar turned out to showa strong, double Balmer emission withaseparation of -1,500 km/s of bothemission peaks. This is typical forcataclysmic binaries with high orbitalinclination and, thus, justified a searchfor eclipses. The rest was good luck:fair weather conditions, the begin of

40