COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                      Number 3378

                                        Konkoly Observatory
                                        Budapest
                                        4 October 1989
 
                                        HU ISSN 0374 - 0676


          IS HR 6754 A DELTA SCUTI STAR AFTER ALL?


HR 6754 (HD 165373, V=6.64, SpT F0 IV-V) is a star which Breger (1989) found
to be located in the delta Sct Instability strip but which he also found to be
non-variable. This conclusion was based on the star being constant to within
0.002 mag over 2.7 hours.

By chance I subsequently used HR 6754 as a check star while studying the
variability of 89 Her, and Table I of Fernie (1981a) reported ten sporadic
observations of HR 6754 over three months which showed constancy to better than
0.02 mag.

However, I have continued to use the star as a check star for 89 Her in an
on-going APT Service program, and in a recent routine examination of the data I
noticed the HR 6754 results seemed less stable than those of check stars for
other variables.

Figure 1 shows the V magnitudes of HR 6754 in 1988 as open circles, while the
plusses represent the V magnitudes of HR 6641, the check star for HD 161796.
The two stars are fairly close together on the sky and so are measured within
minutes of each other on each night, yet it is clear there is significantly
more scatter in the HR 6754 data.

No significant periodicities were revealed when these data were subjected to a
Discrete Fourier Transform analysis, but of course the star's spectral type
suggests any likely period would be a fraction of a day, i.e. at frequencies
much above the Nyquist frequency of these once-a-day observations.

I therefore observed HR 6754 over about two hours on August 24/25 1989, using
HR 6697 as a comparison star. Figure 2 illustrates the results, and
variability is clearly present. A DFT analysis yields a period of 0.072+-
0.008 days and amplitude 0.035 mag, although the unequal heights of the maxima
in Figure 2 suggest that a much longer datastring would likely reveal other
periods as well. In any case, the result is consistent with delta Sct
variability.

The evidence is much against HR 6697 being the variable. Its spectral type of
G2 V militates against detectable pulsation, and its period is much too short
and its lightcurve too shallow for a W UMa system. (The Bright Star Catalog
indicates it to be a spectroscopic binary with components resolved at about 0.1
arcseconds, which leads to a period >=2 years.) Moreover, plotting the
magnitudes of the two stars separately over the two hour run indicates greater
variability in HR 6754. Finally, if HR 6697 were the variable the puzzle of
Figure 1 would remain unanswered since this star is not involved there.


                            [FIGURE 1]

 Fig. 1. Absolute photometry of HR 6754 (circles) and of HR 6641 (plusses)
         in 1988.

                            [FIGURE 2]

 Fig. 2. Differential photometry, HR 6754 - HR 6697, during two hours on
         August 24/25, 1989.


Why the star should have appeared constant at the 0.002 mag level when Breger
observed it is difficult to say. (My own 1981 data are not precise enough to
be more than marginally inconsistent with the present results.) One might argue
that in a multiperiodic variable destructive interference may at times flatten
the lightcurve, but it seems unlikely this could result in constancy to two
millimagnitudes for 2.7 hours. Another possibility, discussed in Fernie
(1981b), la that some stars in this region of the HR diagram are not
continuously variable. The star would clearly repay further observation.


J.D. FERNIE
David Dunlap Observatory
Box 380, Richmond Hill, Ont.
Canada L4C 4Y6

References:

Breger, M. 1969, Ap.J.Supp., 19, 79. [BIBCODE 1969ApJS...19...79B ]
Fernie, J.D. 1981a, Ap.J., 243, 576. [BIBCODE 1981ApJ...243..576F ]
             1981b, P.A.S.P., 93, 333. [BIBCODE 1981PASP...93..333F ]