COMMISSION 27 OF THE I. A. U.
	 INFORMATION BULLETIN ON VARIABLE STARS
		     Number 2648


			 	   Konkoly Observatory
				   Budapest
				   27 December 1984

				   HU ISSN 0374 - 0676



	POSSIBLE LIGHT VARIATION OF COMPARISON STAR FOR
		BY Dra : BD + 51d2410




  The constancy of comparison stars is crucial in photoelectric photometry. In
order to achieve acceptable level of transformation into the international 
systems it is necessary to choose comparison stars of about the same spectral types
as those of the variables. In the case of red stars this causes a considerable
problem since very many of the comparison stars are candidates for some kind of
light variation.

  The particular star that is dealt with here is C1 = BD+51d2410 = HD 172468
(K2) = SAO 31077. When observing BY Dra, most observers use two comparison
stars, the second being C2 = BD + 51d2408 = HD 172268 (K5) = SAO 31070.
Although no detailed investigation has been made to determine the constancy of
these stars over a long time-scale, papers on BY Dra have already mentioned some
problems with its comparisons.
  The first to deal with the problem was Vogt (1981). Vogt compared his 
observations of C1 with those obtained in 1973 (Vogt, 1975) and found some 
discrepancy. He mentioned that "this disparity may well be due to real 
variations of one or both comparison stars."
  In the same year Melkonian et al. (1981) reached the same conclusion. These
authors found some variations in the observations made for C1 and C2 in the
second half of 1975 (see Figure 1 in Melkonian et al., 1981.) and mentioned that
possibly C1 is responsible.
  If we wish to know whether these small variations are real or not, so that
we may avoid the problems which arise when we transform the data obtained in
different places into the same system, we considered it necessary to have
observations using the same telescope and equipment for gives series of
observations.
                         [FIGURE 1a]
                         [FIGURE 1b]

  Our measurements for these stars made at Konkoly Observatory with the 60
cm telescope in 1971 and with the 50 cm telescope in 1983 give a possibility
to investigate this problem.
  In Figure 1 the distributions of the measured magnitudes (DeltaV, DeltaB) are
displayed. The blank area represents all the observations made between 1971
and 1983, the hatched area shows the observations made in 1971 (upper panel)
and in 1983 (lower panel). Although there were fewer observations in 1983 than
in 1971 their distributions is much nearer to Gaussian one.
  Chugainov (1973, 1974) made several observations for these stars. The
histogram of all his observations again shows Gaussian distribution (with 
smaller
standard deviation than ours), but the distribution of his 1971 observations is
far from that.
                          [FIGURE 2]

  The question is : Which of these comparison stars is variable ? When we
plot the light curves for BY Dra itself with respect to C1 and C2 separately,
it can be seen that the scatter is higher when using C1 as comparison 
star in the
year 1971 (Figure 2) whereas in 1983 no difference between the light curves
calculated with C1 and C2 can be found.
  Since C1 is of the spectral type K2 (B-V=1.285, Vogt, 1975) it may show
some kind of activity.





			     K. OLAH
			    Konkoly Observatory
			    Budapest, Hungary

			     M.A. SOLIMAN
			    Helwan Institute of
			    Astronomy and Geophysics
			    Helwan, Egypt





References:

Chugainov, P.F., 1973, Izv. Krimsk. Ap. Obs. Tom. XLVIII. p. 3
	  1974, ibid, Tom. LII. p. 3 [BIBCODE 1974IzKry..52....3C ]
Melkonian, A.S., Olah, K., Oskanian jr., A.V. and Oskanian, V.S., 1981,
   Astrofizika, Tom. 17. 215 [BIBCODE 1981Afz....17..215M ]
Vogt, S.S., 1975, Ap. J. 199. 418 [BIBCODE 1975ApJ...199..418V ]
       1981, ibid. 250. 327 [BIBCODE 1981ApJ...250..327V ]