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Astron. Astrophys. 321, 145-150 (1997)

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5. Other periods?

It has been mentionned in Paper I that on the whole 1950 - 1984 data, only [FORMULA] appeared as a regular sinusoid: [FORMULA] and [FORMULA] show dissymetrical aspects (the light increase of the light curves lasts longer than the light decrease for these periods). A simple test on such discrepancies from regular sinusoidal behavior is to introduce in the analyses a [FORMULA] period simultaneously with period [FORMULA], and to look for an improvement of the fit of the light curves. We did this for every combination of the three principal periods [FORMULA] and their first harmonic [FORMULA].

On the 1987 data the curve fit of the Table 2 is slightly improved by the introduction of [FORMULA], but not by [FORMULA] and/or [FORMULA]. So for these data, only [FORMULA] could be - marginally - different from a sinusoid.

The 1992 data fit is slightly improved by the introduction of [FORMULA], and we can improve it a little further if [FORMULA] is also taken into account. [FORMULA] does not show any significant amplitude improvement and/or rms reduction... So in these data, only [FORMULA] and [FORMULA] could have a shape significantly different from a sinusoid.

So on the whole of our present 1987 and 1992 data, only [FORMULA] can show permanently a light curve different from a simple sinusoid. ([FORMULA] is very probably also the only radial mode).

  1. We do not improve significantly the light curve fit by considering the periods (around 0.084-0.088 d) found by Jerzykiewicz (1993). As shown before, the introduction in our data of these low amplitude ([FORMULA] 1 mmag) periods (quite close to half the principal three periods !) might slightly improve the light curve fit. However, due to our limited string of observations, we cannot derive definitive conclusions about the likely presence or absence of such frequencies in our data.
  2. Jerzykiewicz's (1993) 6.05 d period - or half the orbital period - doesn't give any significant peak and/or data fit improvement in our data.
  3. The period [FORMULA] = 0.139 d , claimed by Jerzykiewicz (1993) as a possible overtone of any of the three principal periods can be detected in our 16 Lac - HR 8766 (2 And) data, in 1987 as well as in 1992: It appears with an amplitude around 1.5 mmag. (We obtain, according to the different period finding methods, periods in the 0.127 - 0.146 d range, with a marked maximum probability around 0.14 d). However a more careful analysis made between comparison stars (i.e. the HR 8766 - HR 8733 and HR 8766 - HR 8708 data) shows that the detected 0.139 d period very probably comes from the star HR 8766 (2 And) itself: the two intercomparison light curves show the same phases and similar amplitudes. A paper is being prepared on this star, as a follow up of the present discovery of its photometric variability.

    If we substract this 0.139 d period with its amplitudes (respectively 1.5 and 1.3 mmag in 1987 and 1992), we obtain Table 4 for the amplitudes of the three classical periods in 16 Lac. These amplitudes are not significantly different from those of Table 2 (a 0.3 to 0.4 mmag difference on the average !): the variations of HR 8766 do not play any role on the conclusions that we will derive on the amplitudes of 16 Lac, up to a confidence level better than 2 mmag.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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