COMMISSION 27 OF THE I. A. U.
	 INFORMATION BULLETIN ON VARIABLE STARS
		     Number 2650

			  	   Konkoly Observatory
				   Budapest
				   2 January 1985

				   HU ISSN 0374 - 0676


       PROBABLE DETECTION OF A VERY SHORT ORBITAL PERIOD
       FOR THE OLD CLASSICAL NOVA CP PUPPIS (1942).




   Spectra of the old nova CP Pup were obtained from May 1 to May 5
1984 with the IDS detector of the ESO 1.5-m telescope of La Silla, Chile.
Those of the first 4 nights in the range 4080-5290 A had a dispersion
of 59 A/mm, while those of the last night in the range 4400-6750 A had
a dispersion of 114 A/mm. Exposure times range from 20 to 40 min.
   After reduction and various corrections the spectra showed signs
of rapid changes on time scales of 1/2 hr. Because of this each of the
exposures, corresponding to the two IDS captures, used to construct our
IDS image files, were considered separately.
   Power spectrum analysis of radial velocities shown  many peaks, because
observations were grouped in runs of the order of 0.1 days for successive
nights. However, as can be seen in Fig. 1, the HeII lambda4686 emission line
radial velocity peaks have themselves a "peak" between periods of 0.05
and 0.07 days, while Hbeta does not so clearly define a best period.

                              Fig. 1. Power spectra of HeII
                             lambda4686 and Hbeta radial velocities.
                             HeII solutions in the frequency
                             range 14.3-19.3 days^-1 are
         [FIGURE 1]
                             given in Table I. Most of the
                             peaks are separated by distances
                             corresponding to integral
                             multiples of the frequency for
                             one day. Some periods are however
                             more probable than others.

           		   Table I

    HeII lambda4686 radial velocity solutions between P= 0.05 and P= 0.07 days
------------------------------------------------------------------------------
     P 	 Comments
 (days)
------------------------------------------------------------------------------
0.06977   H_beta radial velocity better than following, but HeII worse
0.06518   H_beta radial velocities very bad
0.06115   quite good; also given by Chi-square method for HeII equivalent widths
0.05765   not so bad
0.05169   worse for both H_beta and HeII radial velocities
------------------------------------------------------------------------------

                              Fig. 2. Radial velocity
                              curve of HeII lambda4686 (single
                              aperture spectra) emission
      [FIGURE 2]
                              line. Orbital phases are
                              given for P=0.06115 days,
                              T0=JD 2445822.5217.

Various arguments including the scatter of the points, and the phase of
what appears to be an eclipse of H_beta suggest a most probable period of
0.06115 days (see Fig. 2). The equivalent widths of HeII lambda4686 also
appear to have the same period.
   This is the first orbital period of an old classical nova which
would appear to be below the cataclysmic binary period gap (Robinson,
E.L., 1983 IAU Colloquium No. 72, Dordrecht:Reidel, 101, 1).
New observations are needed to settle the question as to whether the
true binary period was detected, rather than the orbital period of a
blob in the disc or some kind of pulsation.



			A. BIANCHINI
			Asiago Astrophysical Observatory,
			I-36012 Asiago (VI), Italy

			M. FRIEDJUNG
			Institut d'Astrophisique, 98 bis,
			Bd Arago, 75014 Paris, France

			F. SABBADIN
			Asiago Astrophysical Observatory,
			I-36012 Asiago (VI), Italy