COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                    Number 2238

                                     Konkoly Observatory
                                     Budapest
                                     1982 December 2

                                     HU ISSN 0374-0676



     REFINEMENT OF THE FREQUENCIES OF PULSATION OF DELTA SCUTI



  New photoelectric observations of delta Scuti taken on 1982 July 14 and 17
using the 0.41 m telescope of the Monash observatory are presented in
Figures 1 and 2, epsilon Scuti (= HR 7032)was used as a comparison star, the 
differential magnitudes being corrected for atmospheric extinction.
  Fitch (1976) compared a solution to the light curve of delta Scuti derived
from Fourier analysis of Fath's data (1935, 1937, 1940) with one obtained
from Fourier analysis of his own data obtained in 1972-73. Differences 
between the two solutions for the larger amplitude frequencies (f0 and fn1)
were small, using these differences Fitch calculated a fractional change in
the fundamental frequency of -1.6x10^-5 in 36 yr but cautioned that such
small differences could be attributed to observational errors.


                            [FIGURE 1]


                            [FIGURE 2]


 Moon and Keay (1982) used observations of a single maximum to refine the
fundamental frequency of delta Scuti, data in Figures 1 and 2 are consistent with
their refined value for the frequency of the fundamental mode of pulsation.
 Using a similar argument to Moon and Keay it is unlikely that our observed
light curves of delta Scuti are shifted by one or more complete cycles from the light
curve predicted by Fitch's solution. These predicted light curves are given
as solid lines in Figures 1 and 2.
 Because Fitch defined an epoch some 40 years prior to his 1972-73 observations 
we found it necessary to choose a new epoch (=HJD 2441800) close to
his observations, and to recalculate the phases accordingly. Data given by
Fitch (1976) are adequately represented by this solution. Only the fundamental
and first nonradial modes (f0, fn1) are known to sufficient precision to refine
their values further. However these two frequencies, along with their second
harmonics (2f0, 2fn1 ) and sum (f0 +fn1 ) describe most of the behaviour of
delta Scuti.
 Adjustment of Fitch's values for the fundamental and first nonradial
frequencies gave the dashed curves in Figures 1 and 2, where f0 =5.160760
cycles day^-1 and fn1 = 5.3540 cycles day^-1 , the estimated errors being
+/- 0.000004 cycles day^-1 and + 0.00004 cycles day^-1 respectively. This was
the closest representation to the data achieved using only f0, fn1, and their
harmonics and combination frequency. Small residuals between our solution for
the light curve and the observations can be accounted for by other frequencies
present and some observational uncertainty.



  Comparing our result for the fundamental frequency with that given by
Fath (1937), f0 = 5.160758, and that given by Fitch for Fath's data (1976),
f0 = 5.160780, it appears that any change in f0 over 45 yr is small and
probably due to observational uncertainty. Considering the precision of
photoelectric photometry and the complicated nature of the light variations
of delta Scuti, the fundamental mode of pulsation appears stable, any 
fractional secular changes are probably less than .000004 in 45 yr.





       D.W. COATES, T.T. MOON, K. THOMPSON and M.L. WINSALL

          Department of Physics, Monash University,
            Clayton, Victoria 3168, Australia





References:
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 ]
Moon, T.T. and Keay, D.M.: 1982, Inform. Bull. Var. Stars No. 2145