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

7. Phases of the different periods

When we compare the light maxima derived from our period analysis with the ephemeris published in Paper I, we obtain the phases of Table 5. If these 1950-1984 ephemeris and their error bars are to be trusted, the eventual errors made on the periods themselves and/or on the times of light maxima can be evaluated from these rms errors, i.e. 0.003, 0.20 and 0.28 in phase respectively for , and .

Table 5. (O-C) phases

We can see there that is still perfectly in phase with the ephemeris, which could be a further confirmation that its amplitude could be modulated by the orbit, the pulsation being "forced" into a more stable value than and 's by the regularity of the orbital period.

On the contrary the two first pulsation modes and are clearly not in phase with the previous ephemeris: The discrepancies observed for these two modes in 1987 and 1992 are about 10 times those observed in the 1950 - 1984 range ! They are much larger than any likely error: the precision obtained on the light curves and on the ephemeris fit shows that only a small fraction of such discrepancies could be due to the too short time basis of the observations in 1987 and 1992. If we assume that only these periods did vary, we would obtain respectively 0.1691675 and 0.1707776 d for and . This means that the amplitude increase observed on could be related to an eventual increase of the period value, which occurred between 1984 and 1987, from 0.1707769 to 0.1707776 d at least (0.1707776 is a lower value assuming that the increase occurred in 1984). The discrepancies between the 1950-1984 ephemeris and the actual 1987-1992 phases could be due

to period - or phases - variations of opposite sign in and between 1984 and 1987, or

to a total extinction of these two modes, followed by a new growth of one or several modes, whose period(s) would show up in our data as an unresolved peak between 0.16917 and 0.17078 d.

Only new observations carried out over more than 18 days would allow one to know.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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