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\IBVShead{4171}{13 March 1995}

\title{ONSET OF PULSATION OF V99 IN M15?}

\begintext
                 
The variable V99 in the globular cluster M15 was discovered by
Mannino on plates obtained in the years 1954-1955 in Asiago
Observatory. From about 200 plates a period of 0.27995 day has
been determined and light curve constructed. It is interesting
to note that this newly discovered RRc variable had been chosen
as comparison star by Bailey in his previous study (Bailey,
1919).

Notni and Oleak (1958) measured the brightness of this star on plates
taken by Guthnick and Prager previously to Mannino's
observations, in the years 1925 and 1933, in Potsdam--Babelsberg
Observatory. On these plates light variation could not be detected.

Brightness measurements were carried out on the plate material
of the Konkoly Observatory (1937-1991). The light curves folded on 
Mannino's period, including the Babelsberg material, show
clear light variation only in the years 1956, 1957 and 1963.

As the plates were originally obtained to study period changes
the temporal distribution of the data is inconvenient and a high
level of noise inherent in the photographic method is present.
Still existence of any definite periodic variation can be
determined. A larger amount of plates were obtained in the years
1938, 1951 and 1990-91. Fourier analysis of the data for these
years was performed.

In the Figures the light curves of the years 1937, 1954 (Mannino's
data), 1956 and 1963 can be seen folded on Mannino's period and
the amplitude spectra with their spectral windows from the years
1938, 1951, 1954 (Mannino) and 1990-91.

The  Fourier analysis of the data of 1925 and 1937 does not show
any frequency but noise. We reanalysed Mannino's (1956)
observations and we could confirm that the period of the star in
1954 and 1955 given by Mannino was correct.

In 1938 a period of 0.28747 day and 0.1 magn amplitude was found.
The period differs from the one given by Mannino by about 0.007 day.
This would be far beyond the rate of period change typical for
RRc stars. So it can be concluded that the deviating period and
the small amplitude of the light curve might show the presence
of a non radial pulsational component. In the year 1951 the
spectrum shows noise with increased amplitude which would
indicate scatter of the data exceeding observational error.

The light curve of 0.40 magn amplitude from 1954 and 1955 defined by
Mannino is that of a typical RRc variable of radial mode
pulsation. The same holds for the Budapest material from the
years 1956 and 1963. From the seventies the limited number of
the data shows only scatter slightly exceeding the photographic
error. At any rate it suggests diminishing amplitude of any of
the periods.
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Figure 1. Light curves from Budapest observations in the years 1937, 
      1956 and 1963 folded on Mannino's period. The light
      curve from 1954 is based on Mannino's observations.
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Figure 2. Amplitude spectra with their spectral windows from the
      years 1938, 1951, 1954 (from Mannino's data) and 1990-91.     
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In the years 1990 and 1991 the structure of the spectra in spite
of the very low amplitude might indicate some oscillation inside 
the star.

Time and more accurate photometry can only solve the problem of this
unique cluster member. It is suggested that the starting of pulsation
has been caught.

This study was partly supported by Hungarian OTKA grant I-3-829.
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\beginauthors
\authorsrightcol
K. BARLAI and 
B. SZEIDL
Konkoly Observatory, Budapest
Hungary
\endauthors

\references

Bailey, S.I., 1919, {\it Harv. Ann.}, {\bf 78}, Part 3 

Mannino, G., 1956, {\it Memorie Soc. Astr. Ital.}, {\bf 27}, 263

Notni, P., Oleak, H., 1958, {\it Astron. Nachr.}, {\bf 284}, 49
		
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