COMMISSION 27 OF THE I. A. U.
          INFORMATION BULLETIN ON VARIABLE STARS
                      Number 2758

                                       Konkoly Observatory
                                       Budapest
                                       10 July 1985

                                       HU ISSN 0374 - 0676



            THE DOUBLE-MODE CEPHEID BQ Ser



   Wenzel (1966) and Szabados (1975) observed the double-mode
Cepheid BQ Ser in the UBV system, but did not publish the data.
Because only twelve such stars are known to exist, it is important
that a reddening be accurately determined so that the star can be
placed in the H-R diagram. For this purpose, we observed BQ Ser in
the Johnson UBV and Kron-Cousins VRI systems with the 0.5-m
reflector of the South African Astronomical Observatory (Table I).
As standards we used HD171367 (V = 7.644, B-V = 1.165, U-B = 1.009,
V-R = 0.624, V-I = 1.215) and HD172365 (V = 6.373, B-V = 0.794, U-B
= 0.502, V-R = 0.442, V-I = 0.856). The magnitudes and colours of
these two stars were obtained by observing E-region standards.
   Power spectrum analysis confirmed the periods already obtained
for this star: we find P0 = 4.283 and P1 = 3.012 days. Because the
older data are not published, these periods could not be refined.
Fourier analysis gives the following amplitudes and phases:

m      m0      A0       phi0       A1     phi1
V      9.499  0.183    -0.249    0.107   0.238
B-V    1.446  0.079    -0.221    0.047   0.267
U-B    1.039  0.075    -0.201    0.047   0.283
V-R    0.876  0.045    -0.198    0.026   0.290
V-I    1.732  0.074    -0.212    0.044   0.267

where m = m0 + A0cos2Pi(t/P0+phi0) + A1cos2Pi(t/P1+phi1) and the time t
is measured from JD2445000.000.
   Using the relations given in Dean, Warren and Cousins (1978),
we obtain <B-V>0 = 0.626, <V-I>0 = 0.656. The star is very heavily
reddened with EB-V = 0.82. Caldwell's (1983) PLC relationship
gives MV = -2.90. This places BQ Ser very near the red edge of
the instability strip and shows that it is considerably cooler than
any of the other double-mode Cepheids.





                           Table I


                  Observations of BQ Ser.

     HJD-2445000  V       B-V     U-B     V-R     V-I
        823.660  9.581   1.502           0.887   1.758
        825.663  9.183   1.314           0.797   1.598
        828.662  9.534   1.447           0.867   1.731
        829.669  9.314   1.338           0.834
        859.624  9.265   1.337           0.849   1.659
        860.623  9.549   1.481           0.925   1.788
        861.617  9.635   1.497           0.922   1.791
        862.620  9.516   1.444           0.911   1.755
        866.623  9.757   1.528           0.974   1.853
        892.425  9.543   1.452           0.882   1.747
        893.420  9.470   1.435   1.029   0.843   1.677
        897.410  9.506   1.416   0.992   0.860   1.714
        922.375  9.557   1.442   1.023   0.875   1.743
        923.358  9.407   1.408   1.016   0.840   1.698
        924.395  9.451   1.422   0.995   0.859   1.714
        925.380  9.497   1.454   1.055   0.889   1.737
        926.357  9.649   1.508   1.122   0.902   1.799
        927.324  9.557   1.450   1.015
        929.373  9.548   1.485   1.076   0.891   1.773
        930.276  9.759   1.575   1.149   0.921   1.829
        936.307  9.407                   0.848   1.693
        939.323  9.776   1.558   1.149   0.903   1.792
        940.316  9.233   1.320   0.942   0.770   1.580
        957.248  9.576   1.451   1.035   0.885   1.763
        958.266  9.223   1.327   0.950   0.805   1.630





           L.A. BALONA and C.A. ENGELBRECHT
           South African Astronomical Observatory
           P.O. Box 9
           Observatory 7935
           Cape
           South Africa

References:

Caldwell, J.A., 1983. Observatory, 103 244. [BIBCODE 1983Obs...103..244C ]
Dean, J.F., Warren, P.R., and Cousins, A.W.J., 1978. Mon.Not.Roy. Astr.Soc.
      183, 569 [BIBCODE 1978MNRAS.183..569D ]
Szabados, L., 1975. "Multiple periodic Variable Stars" (ed. W.S. Fitch),
      Budapest, 159. [BIBCODE 1976mpvs.conf..159S ]
Wenzel, W., 1966. M.V.S., 4, 25.