COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 2145

                                   Konkoly Observatory
                                   Budapest
                                   1982 May 17

                                   HU ISSN 0374-0676


      REFINEMENT OF THE FUNDAMENTAL FREQUENCY OF PULSATION
                   OF DELTA SCUTI


    Previously unpublished photoelectric magnitudes for delta Scuti
(= HR 7020 = HD 172748) are presented in Figures 2, 3 and 4. These results
were taken at Mt. Arapiles in Victoria on 1977 August 23 using a 20 cm
telescope, an uncooled 1P21 photomultiplier tube and standard U,B,V filters.
A number of standard stars were also observed; atmospheric extinction
coefficients were derived (k_V=0.17 mag/air mass; k_B=0.32 mag/air mass;
k_U=0.61 mag/air mass) and adequate transformations to the standard UBV system
established.
     epsilon Scuti (= HR 7032 = HD 173009) was used as a comparison star;
observed magnitude differences were corrected for differences in air mass
between the variable and comparison stars then transformed to the standard
UBV system. Photoelectric V magnitude, B-V and U-B colour indices for 
epsilon Scuti
are given by Cousins (1964); using these values and the corrected magnitude
differences, the average magnitude and colour indices for delta Scuti were 
calculated to be V = 4.73, B-V = 0.36 and U-B = 0.10, in excellent agreement
with average values given by Cousins and Stoy (1962). HR 7007 (= HD 172348)
was used as a check star for V band measurements; the probable error in a
single observation was determined to be +- 0.01 magnitude.
     These data were not of sufficient precision or extent to justify
analysis using Fourier techniques. However a maximum is clearly observed in
all three bands at about HJD 2443379.05. Solutions for the light variation
of delta Scuti are given by Fitch (1976). Using the frequencies determined by
Fitch from analysis of b-magnitude measurements, we calculated the predicted
light curve of delta Scuti at the time of our observations (see Figure 1.).
Dashed lines in Figures 2, 3 and 4 represent this predicted light curve
shifted 0.02 day to the left (i.e. to earlier times). Despite the large
scatter in our observations, this shifted curve is a good fit to results
in all three bands.
    Fitch (1976) compares the fundamental frequency of pulsation (f_0)
for delta Scuti calculated from his b-magnitude measurements (f_0=5.16070 cycles/day)
with that calculated using Fath's (1935, 1937, 1940) white light measurements
(f0=5.16078 cycles/day) and suggests that a secular change in f_0 of -0.000016
in 36 years may have occurred. He cautions that such a small difference
can be explained in a number of ways including observational errors. Our
observations of delta Scuti were taken 1542 days after Fitch's data for JD 2441837.

                          [FIGURE 1]
                          [FIGURE 2]
                          [FIGURE 3]
                          [FIGURE 4]

This corresponds to about 8000 cycles of the fundamental mode of pulsation
of delta Scuti. Considering the extensive data collected by both Fath and
Fitch and the excellent agreement in the fundamental frequency calculated
from these separate sets of data, it is unlikely that our observed maximum
could be shifted by one or more complete cycles from the predicted light
curve. Hence agreement between the predicted light curve and Fitch's
data for JD 2441837 (Fitch, 1976), and displacement of our observed
maximum from the predicted maximum on JD 2443379 (0.02+-0.004 day) would 
indicate
a secular change in f_0 of +0.000013 in 4 1/4 years. It appears unlikely
that these apparent changes in f_0 are primarily a result of secular changes
in the fundamental frequency, but can be largely attributed to observational
errors.

     If this is the case then we can refine the fundamental frequency
of pulsation of delta Scuti using the difference in time between our predicted
and observed maxima. Such a procedure yields a fundamental frequency for delta
Scuti of 5.160767+-0.000013 cycles/day and provides a useful basis for 
further observations to measure secular changes in f_0.
     T. Moon is supported by a Commonwealth Government Postgraduate
Research Award; M. Keay is a fourth year student in the department of
Mechanical. Engineering, Monash University. Thanks are extended to Professor
J. A. Thomas, RAAF Physics, University of Melbourne for his supervision
at the time these observations were made.



                       T. T. MOON and D. M. KEAY
                    Department of Physics, Monash University,
                      Clayton, Victoria 3168. Australia.

References:
Cousins, A.W.J.: 1964, Mon. Not. R. Astr. Soc. Sth, Africa, 23, 175 [BIBCODE 1964MNSSA..23..175C ]
Cousins, A.W.J. and Stoy, R.H.: 1962, Roy. Obs. Bull. No. 64 [BIBCODE 1962smer.book.....C ]
Fath, E.A.: 1935, Lick Obs. Bull., 17, 175, [BIBCODE 1935LicOB..17..175F ]
Fath, E.A.: 1937, Lick Obs. Bull., 18, 77 [BIBCODE 1937LicOB..18...77F ]
Fath, E.A.: 1940, Lick Obs. Bull., 19, 77 [BIBCODE 1940LicOB..19...77F ]
Fitch, W.S.: 1976, IAU Colloq. No. 29, 167 [BIBCODE 1976ASSL...60..167F ]