COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 2103

                                Konkoly Observatory
                                Budapest
                                1982 March 8

                                HU ISSN 0374-0676





        PHOTOELECTRIC PHOTOMETRY OF Ap STARS IN THE
      GALACTIC CLUSTER NGC 2516: PRELIMINARY RESULTS



    The galactic cluster NGC 2516 is known to contain many Ap stars
of all peculiarities (Abt and Morgan 1969; Dachs 1972; Hartoog 1976;
Maitzen 1981 etc.) and so is of particular interest for the study of
the evolution of such stars. Four of these, which had a slightly
greater dispersion than others in the Geneva photometric measures,
were systematically checked for variability during the years 1981-82
at La Silla Observatory, with the 70cm Swiss telescope. These are
Cox 15 (HD 65987), 24 (HD 66318), 38 (CpD-60d 00981) and Cox C
(CpD-60d 00978). HD 65950, which is a Mn star, does not vary 
significantly and was used as a comparison star together with 
CpD-60d 00982.

    Cox 15 (SiSr) has a peak-to-peak amplitude of no more than 0.025m
in [U-B] and of 0.017m in V. Twenty-one measurements of this star
have been made between February and April 1981, each one consisting in
the sequence C_1-V-C_2-V-C_2-V-C_1 (C1 and C2 are the above-mentioned 
comparison stars). The most likely period seems to be 1.41 days, with a
double wave in [U-B], but the V lightcurve is very scattered and more
measurements are being made to clarify this point.

   Cox 24 (Si) shows no detectable variation: eight measurements
show a standard deviation of only 0.0024m in V, which surely is close
to their intrinsic scatter.
                     
                          [FIGURE 1]

   Cox 38 has been measured nine times in 1981 and shows no intrinsic 
variation, the r.m.s. deviation being 0.0026m in V; however, one
measurement yields a magnitude of 9.65 instead of 9.51, and the three
magnitudes of the sequence are monotonically increasing, while the
colours remain completely normal. This suggested an eclipse, and
the star was observed again in December 1981 and January 1982 in order
to find other minima. Two were indeed found (see the figure, where
the magnitude differences between Cox 38 and CpD-60d982 are represented) 
and one of them may well be a secondary minimum; however, as it
has been measured mainly during twilight, and although the sky was
measured simultaneously with a double-beam photometer, this still has
to be ascertained. A rough estimate of the half-period, or of the
period itself can be made and lead to the following ephemeris:

HJD (Min.) = 2444985.877 + 3.157 E
                  -+.005  -+.007

More measures will be made in order to complete the lightcurve.
The absence of intrinsic variation may imply that the magnetic and
rotation axes are parallel, if the "spots" are located near the 
magnetic poles and if the rotation axis is normal to the orbital plane.

   Cox C (Si) has a very significant variation of 0.06m in [U-B]
and of 0.03m in V. Its most probable period is 1.81 days, and the
V lightcurve is double-waved.




P. NORTH                                F. RUFENER
Institut d'Astronomie de l'Universite   P. BARTHOLDI
de Lausanne et Observatoire de Geneve   Observatoire de Geneve
CH-1290 Chavannes-des-Bois              CH-1290 Sauverny
Switzerland                             Switzerland


References:

Abt H.A., Morgan W.W.: 1969, Astron. J. 74, 813  [BIBCODE 1969AJ.....74..813A ]
Dachs J.: 1972, Astron. Astrophys. 21, 373 [BIBCODE 1972A&A....21..373D ]
Hartoog M.R.: 1976, Astrophys. J. 205, 807 [BIBCODE 1976ApJ...205..807H ]
Maitzen H.M.: 1981, Astron. Astrophys. 96, 151 [BIBCODE 1981A&A....96..151M ]