COMMISSIONS 27 AND 42 OF THE IAU
            INFORMATION BULLETIN ON VARIABLE STARS
                         Number 3851

                                             Konkoly Observatory
                                             Budapest
                                             26 February 1993

                                             HU ISSN 0374 - 0676


            TAU Cas: A VARIABLE STAR AFTER ALL?

The bright star Tau Cas (HR 9008=HD 223165; K1 IIIa) has been commonly used as
a comparison star for the variable supergiant Rho Cas.  It is a controversial
choice:  the New Catalogue of Suspected Variable Stars lists it as variable
with a possible amplitude of 0.3 magnitudes.  Some observers (e.g., Leiker &
Hoff, 1988) have claimed recent variability, while others (Percy, 1985;
Halbedel, 1989) have seen none.

Consequently, this author has continued to use Tau Cas as a comparison star for
observations of Rho Cas.  However, when the past two observing seasons' BV data
has been examined, it became clear that there had been a slight change in its
behavior.  All observations were obtained with the 0.6-m.  telescope of the
Corralitos Observatory and two photometric systems, one based on an EMI 9924A
tube and the other on an R4457.  As comparison star, HR 9010 (K3 IIb; V=5.510;
B-V = +1.650; U-B= +1.810:  V-R= +1.290) was used.

Table 1 lists the magnitudes which have been obtained since Halbedel (1989);
Figure 1 shows them graphically.  It is clear that there has been a slight
brightening in the last two observing seasons, as well as a gradual reddening.
The extent of brightening at present is estimated to be 0.05 V magnitudes,
which is in agreement with Percy's (1985) contention that if the star varies,
it is only by no more than several hundredths of a magnitude.

Is this a real brightening?  The astute reader will have noted that two
different photometers were used to obtain the data.  However, extreme care was
taken with the observations of standard stars to insure that the
transformations between the two systems were such that consistency of magnitude
and color was maintained.  Also, despite a large observing program of other
stars (both early and late in spectral type), this effect was noted in no other
case.  Nor is it likely to be a transient atmospheric effect (e.g., Mt.
Pinatubo) since it was not found in other data.

One final caution is necessary:  the observations of Tau Cas have been made
with respect to only one comparison star.  It is possible that HR 9010 is the
variable.  However, that star has no history of variability or suspicions
thereof.  Therefore, at this point, the variability of Tau Cas must be
considered as suggestive until proven by observations made with two comparison
stars.  At the very least, this observer will in future replace Tau Cas as one
of the comparisons for Rho Cas and will observe it as a potential variable in
is own right until this question is settled.  However, because of the relative
stability of Tau Cas in the past, all previously published magnitudes for Rho
Cas by this author still stand.

The author wishes to most gratefully acknowledge a Theodore Dunham, Jr.  Grant
of the Fund for Astrophysical Research for the purchase of photometric
equipment.


                              [FIGURE 1]

 Figure 1. Magnitudes and colors for Tau Cas. The top diagram shows V,
           the bottom B-V. Julian Date is JD - 2440000.


                        Table 1

            UBVR magnitudes for Tau Cas
          Since only one star was used for comparison purposes,
            there are no standard errors

 Julian Date    V       B-V         U-B     V-R
  (2440000+)
  7835.70139    4.854   +1.084
  7861.60486    4.851    1.102
  7878.64374    4.857    1.111
  7896.61250    4.860    1.114
  7917.59236    4.857    1.103
  8122.81875    4.873    1.093
  8133.83541    4.875    1.097
  8159.78611    4.880    1.098
  8176.69028    4.861    1.116
  8178.72639    4.872    1.116
  8201.68263             1.111
  8204.61111    4.840    1.116
  8228.62917    4.891
  8244.63819    4.870    1.092
  8535.74861    4.848    1.108
  8546.70833    4.840    1.107
  8562.69444    4.820    1.110
  8597.67292    4.829    1.116
  8620.59583    4.839    1.102
  8636.59028    4.814    1.126
  8637.59236    4.822    1.134
  8800.93056    4.816    1.128
  8842.89306    4.817
  8898.78958    4.839    1.108     +1.001   +0.995
  8932.77292    4.828    1.131      1.049    0.965
  8951.70903    4.831    1.117      1.029    0.999
  8973.65694    4.823



                                                ELAINE M. HALBEDEL
                                                Corralitos Observatory
                                                P.O. Box 16314
                                                Las Cruces, NM
                                                U.S.A. 88004

References:

Halbedel, E. M., 1989, IBVS, No. 3394
Leiker, P. S. & Hoff, D. B., 1988, IBVS, No. 3176
Percy, J. R., 1985, Jour. AAVSO, 14, 52 [BIBCODE 1985JAVSO..14...52P ]