COMMISSION 27 AND 42 OF THE IAU
                   INFORMATION BULLETIN ON VARIABLE STARS
                               Number 3822

                                                         Konkoly Observatory
                                                         Budapest
                                                         4 January 1993

                                                         HU ISSN 0374 - 0676


                  NEW RAPID PHOTOMETRY OF BQ Cnc USING CCDs


   Delta Scuti stars are a well known type of variable stars placed in the
instability strip of the HR diagram. Periods are normally between half an hour
and five hours. Although many studies have been made on Delta Scuti stars (see
e.g. Belmonte et al. 1991) both theoretical and observational, a lot of
questions remain unsolved. Kappa mechanism is commonly adopted as the
excitation mechanism (Chevalier 1971), but the fact that a concrete mode
appears and disappears, after its lifetime has passed, is not completely
understood. Only a few modes have been detected for each star and in some
cases they may disappear before the star is observed again.
   In this communication we present the results of new observations of the
Delta Scuti BQ Cnc. Variability of this star was discovered by Breger
(1973), who reported a period of 0.074+-0.014 days, with an amplitude of 0.1
magnitude, corresponding to a peak of 150 microHz in the amplitude spectrum. Only
three hours of observation were obtained in this occasion.
   Our BQ Cnc observations were conducted on 19-22 January 1991 at the 1m
Jacobus Kapteyn Telescope at 'El Roque de los Muchachos', (La Palma, Canary
Islands, Spain). The detector was a CCD camera, with 400x590 pixel GEC
detector. We defocused the stars to spread the photons over a large number of
pixels without reaching saturation, as we wanted to get as many photons as
possible in order to increase the signal to noise ratio. We only obtained
three nights of data because of bad weather, with a total of 1296 integrations
of 50 seconds each, along 80 hours of observations (see Table I).


               Table I: Journal of observations:

date   series longitude  number of integrations   atmospheric quality
(1991)     (hours)                                   (1=perfect)

19 Jan      7.1                 450               0.8(thin cirrus)
20 Jan      3.5                 250               0.5(dust)
22 Jan      9                   596               1.


   Aperture photometry was used to reduce 1296 CCD frames because of
defocusing. We used the aperture photometry part of DAOphot routine (Stetson
1987), although ours was not a crowded field. We applied an Iterative Sinewave
fitting algorithm (Ponman 1981) to compute amplitude spectra of residual series
since it has been proved to be a good method of harmonic analysis with
non-homogeneous sampling (Belmonte et al 1991). Results are shown in Figure 1
(a,b). Three peaks can be distinguished in the amplitude spectrum:


Frequency      Amplitude    Phase     Period
   (mHz)       millimag.              hours

  0.025         11.9      -1.0297    11.11
  0.119          4.3       1.7451    2.334
  0.174          4.1       0.3314    1.596

    The first peak, corresponding to 11.11 hours is probably a harmonic of a
day periodicity (1/2 day) due to minor atmospheric transparency and color
effects, caused by the comparison star we used. This comparison star was
fainter than BQ Cnc and with a different color index (it was the best we could
get in the field), so corrections of the transparency are not as accurate as we
would have desired. We have eliminated this effect by subtracting the peak
from the amplitude spectrum. Pre-whitening of the other two frequency peaks
was applied as well. Pre-whitened spectrum is presented in Figure 1c. Maybe
some other peaks in the spectrum could be considered as possible real peaks.
However they are too close to noise level to be confident on them. Ratio
between these two possible oscillation frequencies is 0.68. With a resolution
in the spectrum of 0.002 mHz, it could be the ratio Nu_1,2/Nu_1,1=~0.73.
Besides, there might be a 1d^-1 aliasing misinterpretation.


                                [FIGURE 1]

 Figure 1: a) and b) Amplitude spectra of BQ Cnc, c) pre-whitened spectrum.


   With these new observations, it seems possible that the frequency peak at
150 microHz discovered by Breger were, in fact, two separate peaks that he could
not resolve due to the poor sampling. Resolution for this series in the
amplitude spectrum was 92 microHz, and the separation between peaks found in this
analysis is 68 microHz.


                                        Immaculada VIDAL
                                        Juan A. BELMONTE
                                        Esther LEUNG(*)
                                        Instituto de Astrofisica
                                        de Canarias, Tenerife, Spain

(*) Summer research student at IAC, University of London, England



References:

Belmonte, J.A. et al, 1991, Astron. Astrophys., 246, 71. [BIBCODE 1991A&A...246...71B ]
Breger, M., 1973, Astron. Astrophys., 22, 247. [BIBCODE 1973A&A....22..247B ]
Chevalier, C., 1971, Astron. Astrophys., 14, 24. [BIBCODE 1971A&A....14...24C ]
Ponman, T., 1981, M.N.R.A.S., 196, 583. [BIBCODE 1981MNRAS.196..583P ]
Stetson, P.B., 1987, P.A.S.P., 99,191. [BIBCODE 1987PASP...99..191S ]