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

## 4. Amplitudes

On the available 1987 and 1992 data we decided to force the long- known three periods, on the basis that they have been stable for 35 years since the first reliable photometric observations, i.e. since the very beginning of the 1950's (Paper I). So we introduced the most recent and precise determination of these periods ( = 0.1691678 d, = 0.1707769 d, = 0.1817331 d, Paper I) to derive and analyze in our data their respective amplitude variations.

Given the three above periods, we obtain by a least squares fit of sine curves through the 1987 and 1992 data the amplitudes (half variation light range) of Table 2 in the filter b (or its equivalent filter #5), in millimagnitudes (mmag). The Fig. 3 shows the fit obtained for 1992's data. Table 2. Amplitudes in #5 or b filter, in millimagnitudes (mmag). Fig. 3. The whole 1992 campaign in the b filter. Vertical ticks are 1 mmag apart. We superimposed the fit obtained with the three main periods, which leads to the 1992 amplitudes of Table 2.

(NB: All our 1987 and 1992 observations are "out of eclipse", according to the ephemeris given by Pigulski and Jerzykiewicz (1988)).

If we only take into account the "Danish" photometer data of 1992, we obtain amplitudes of 6.3, 9.9 and 6.6 mmag for , and (instead of 5.2, 11.1 and 6.2 for the whole 1992 data). These discrepancies - about 1 mmag on the amplitudes - give an order of magnitude of the possible error bars.

Apart from the rms errors quoted in Table 2, we can have an idea of the actual precision on the amplitudes if we look for periods in the data of the second comparison star (HR 8708 - HR 8766): We find 1.5 mmag amplitudes with 4 mmag rms So we can think the error bar to be at most 2 mmag on the amplitudes of Table 2. On the data, one can obtain the color/amplitude ratio between UV (filter u or #4) and visible (filter y for example, or blue #5): (Au - Ay)/Ay. This ratio, as well as that of the amplitudes of radial velocities to magnitudes RV/ m is an indicator of the pulsation mode, as already shown by several authors.

The color/amplitude ratio for being somewhat higher than those of or , one can infer that these pulsations might be of a different kind: and are non radial modes, according to Fitch (1969) and Jerzykiewicz (1993). is very probably a l=2 mode, and could be a l=1 mode (Paper I), although this latter mode is rather unusual in Cep pulsators (Heindericks et al.,1994). Although the precision attained on the color/amplitude ratios is not very high, we obtain here for ratios (0.41 and 0.55) that are in the same range as previous determinations (0.42 in Jerzykiewicz, 1993, and 0.62 in Paper I). is very probably radial and could be the fundamental; Table 3 shows that and are unlikely to be the product of a unique mode splitted by rotation ! (Some previous analyzes had attributed NR modes to and , and a radial mode to : see Balona, 1985, Pigulski and Jerzykiewicz, 1988). Table 3. Color/amplitude ratio in 1987 and 1992 (uvby data)    © European Southern Observatory (ESO) 1997

Online publication: June 30, 1998 