COMMISSION 27 OF THE I. A. U.
           INFORMATION BULLETIN ON VARIABLE STARS
                      Number 2751

                                          Konkoly Observatory
                                          Budapest
                                          21 June 1985

                                          HU ISSN 0374 - 0676


THE OLD-NOVA GK PER (1901) : EVIDENCE FOR A TIME-DELAY BETWEEN ITS X-RAY
                    AND OPTICAL OUTBURSTS.




   One of the most intriguing peculiarities of the old-nova GK Per is
its "dwarf nova-like" behaviour (Sabbadin and Bianchini, 1983, Astron.
Astrophys. Suppl. Series 54, 393).
   A comparative analysis of X-ray, UV, and optical data from the nova
during the outburst phenomena revealed the existence of a time-delay
between the X-ray and the optical light curves. This is shown in Fig. 1
which refers to the 1978 outburst, the only one for which an extended
X-ray monitoring of the old-nova is available ( King et al., 1979,
Monthly Notices Roy. Astron. Soc. 187, 77P). In this figure the X-ray
data are as in King et al. (1979) whereas the optical ones have been
kindly communicated to us by Janet Mattei, director of the AAVSO.


                             [FIGURE 1]

 Fig.1. The optical (top panel) and the X-ray (bottom panel) light
 curves of GK Per during the 1978 outburst. Comments on the X-ray data 
 points are given by King et al. (1979).
 The dashed line refers to what seems to be a steep rise.


   Fig.1 clearly shows that the X-ray phenomenon started about 30 days
before the optical one, which means that the former is not simply a
consequence of an enhanced Roche-Lobe overflow from the secondary. This
might also explain the presence of high excitation lines of OV lambda1371,
[ArIII] lambda3110, [ArIV] lambdalambda2854,2869 and [NeIV] lambda2423 in
the IUE spectra of the nova taken during the early rise to the 1981 
maximum and their


absence (or weakness) at light maximum and during the decline (Bianchini
et al., 1985, submitted to Astron. Astrophys.).
   We wish to note here that the time-delay between the X-ray and the
optical outburst is quite a common phenomenon in X-ray novae (Ciatti
et al., 1977, Astron. Astrophys., 56, 311).
   It seems to us that a more systematic X-ray survey of the nova will
be of paramount importance for understanding its outburst activity.
Actually, only a continuous X-ray monitoring will allow the study of the
very beginning of the phenomenon.





                       A. BIANCHINI
                       F. SABBADIN

                       Asiago Astrophysical Observatory
                       I-36012 Asiago (VI), Italy