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Astron. Astrophys. 346, 487-490 (1999)
3. Fourier parameters
As a further step, we fitted the V magnitudes by means of
the formula
![[EQUATION]](img9.gif)
where f is the frequency, measured in cycles per day
(cd-1 ). From the least-squares coefficients we calculated
the Fourier parameters (in particular
) and
(in particular
). These parameters are reported in
Table 1; the mean magnitude of OGLEGC 29 is assumed from
Kaluzny et al. (1996) as the values listed in the electronic table are
shifted up by 2.5 mag. The period values obtained from the
least-squares routine are listed, but they do not differ greatly from
those reported by Kaluzny et al. (1996, 1997).
![[TABLE]](img20.gif)
Table 1. The phases differences ( ) and amplitude ratios ( ) obtained from the Fourier decomposition are reported together with period values. The half-amplitude of the light variation is also reported. In 12 cases the term (i.e. the first harmonic) was not detected.
Typical error bars are 0.33 rad
for the values and
0.05 for the
ones. Note that the amplitudes
quoted hereinafter are those of the cosine terms, i.e. the
half-amplitude of the light variation. No significant
term could be evidenced in 12 cases
(OGLEGC 7, 24, 34, 35, 37, 40, 46, 59, 60, 63, 66, 70). For these
stars the light curves do not deviate appreciably from a sinusoid:
that means that if a 2nd-order fit is forced on the data, the error
bar on the amplitude of the 2f term is larger than the
amplitude itself. Following the same criterium, in 15 other cases the
fit was stopped at the 2nd-order, in 6 cases at the 3rd and in two
cases at the 4th.
Fig. 2 shows the plot: the stars
have been subdivided into three groups according to their amplitude
and different symbols have been used. As can be noticed, there is a
well defined trend in the diagrams. Moreover, the
values (open squares in Fig. 2)
related to the galactic stars CY Aqr, ZZ Mic (Antonello et al. 1986)
and V831 Tau (Musazzi et al. 1998) are in excellent agreement with
those related to stars in Cen.
![[FIGURE]](img37.gif) |
Fig. 2. The progression for short period stars in Cen. Different symbols for different amplitudes: filled dots for mag, open dots for mag, crosses for mag. The three open squares on the right side denote the three galactic stars CY Aqr, ZZ Mic and V831 Tau. Error bars are reported for the individual cases discussed in the text
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There are some interesting cases:
OGLEGC 26 - The light curve is noisy (rms residual
0.033 mag), but its shape looks quite strange, with a descending
branch steeper than the ascending one (Fig. 3, upper panel). The
reality of the asymmetry is even more obvious when considering the
mean light curve (Fig. 3, lower panel). In the Galaxy, there are two
high-amplitude Sct stars with a
similar light curve: V1719 Cyg (Poretti & Antonello 1988) and V798
Cyg (Musazzi et al. 1998). Both these stars have a double-mode nature.
Since the number of measurements of OGLEGC 26 is adequate (231 on
50 nights), a second period should be revealed by the frequency
analysis, but we failed to find it.
![[FIGURE]](img39.gif) |
Fig. 3. The star OGLEC 26 shows an asymmetrical light curve, with a descending branch steeper than the ascending one (individual points, upper panel ). This asymmetry is even more obvious in the mean light curve (lower panel )
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OGLEGC 29, 36, 39 and 62 - There are a few cases where
the values seem to deviate from the
progression described by the others (Fig. 2). When considering the
error bars, the values of the light
curves of OGLEGC 29 and 39
(3.72 0.66 rad and
4.02 0.82 rad, respectively) are only
marginally deviating; in the case of OGLEGC 62
(
=4.13 0.56 rad) the line is just
within the error bar of the related point. This discrepancy can be
explained by observational scatter, since the amplitude of the
terms is very small. Moreover, the
error bars may be optimistic since they are obtained from the formal
error propagation. However, we note that the highest value
(4.65 0.49 rad for OGLEGC 36) is
the more reliable one and is farther than
3 from the others.
© European Southern Observatory (ESO) 1999
Online publication: May 21, 1999
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