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Astron. Astrophys. 350, 566-570 (1999)

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3. Analysis

In Fig. 1 we plot the nightly mean V magnitudes vs. HJD for all observations from Table 3. of OT Gem (middle panel) and the check star HR 2858 (bottom panel). The larger scatter in the check star in the last two seasons of Hvar measurement is due to a bad weather conditions. From HJD 2444977 to 2446259 the brightness of OT Gem moderately decreases. After the period not covered with measurements the star passed through three active phases. The first sudden brightening was detecteded by the Hipparcos satellite (HJD from 2448705 to 2449046), the second one by the measurements from La Luz and Hvar Observatories (HJD from 2450087 to 2450195) and the third one from Hvar Observatory (HJD from 2450863 to 2450910). Between the second and third active phase, OT Gem reached its minimum brightness as reported by Ferro et al. (1998).

[FIGURE] Fig. 1. Upper panel - H[FORMULA] intensities of OT Gem vs. time (solid circle - Andrillat (1983), open circles - Dachs et al. (1986), solid box - Doazan et al. (1991). Middle panel - The nightly means of photometric measurements of OT Gem vs. time (solid circles - Hvar, open circles - La Luz, solid diamonds - Hipparcos, solid triangles - Merate, pluses - San Pedro Mártir). Bottom panel - The nightly means of the check star HR 2858

During the first two outbursts, the brightness of OT Gem obviously exhibited cyclic variations. Ferro et al. (1998) formally derived the 83 and 45 day periods from the full set of individual observations or the 77 and 48 day periods from the nightly means. They interpreted these "periods" as characteristic times of the long-term variations but not as a strict periodicity.

To check on their results, we used all nightly means of La Luz, Hvar and Hipparcos V magnitudes and carried out the frequency analysis, taking into account measurements from the active phases only. We removed the long-term trend by the program HEC13 (Harmanec, private communication), based on the smoothing technique derived by Vondrak (1969) and Vondrak (1977), and using the smoothing parameter [FORMULA]. The prewhitened values were submitted to Stellingwerf 's (1978) PDM period search technique as well as to Deeming (1975) Fourier analysis over the frequency range from 0 to 0.05 c d-1 and with a frequency step of [FORMULA]. The PDM method detected as the best frequences the values of 0.01388 c d-1 (72.[FORMULA] 0461) and 0.01391 c d-1 (71.[FORMULA] 8907) for the bin/cover structure (5,2) and (10,4) respectively, whereas the Fourier analysis gave 0.01295 c d-1 (77.[FORMULA] 2201). In Fig. 2 we plot Stellingwerf 's [FORMULA] statistics. We believe that such variations are cyclic rather than strictly periodic. From Fig. 1 in Ferro et al. (1998) it is obvious that the oscillations of OT Gem are non-sinusoidal, so we prefer the 71.[FORMULA] 8907 period as the best representation of the cyclic variations. The corresponding phase diagram is in Fig. 3.

[FIGURE] Fig. 2. Stellingwerf 's [FORMULA] statistics of OT Gem for the bin/cover structure (10,4)

[FIGURE] Fig. 3. Phase diagram of the light variations of OT Gem for 71.[FORMULA] 8907 period. The prewhitened V magnitudes are plotted on the Y-axis. The following symbols are used: solid circles... Hvar, open circles... La Luz, solid boxes... Hipparcos

The presence of the 0.[FORMULA] 15 short-term variations announced by Ferro et al. (1998) was already questioned by Bozic et al. (1982) and Poretti (1982) but they did not carry out the frequency analysis because of their limited data sets. Ferro et al. (1998) reported that they were not able to find a short term period from their frequency analysis. Because of the large light variations in the later seasons, we restricted our analysis to the "quiet" period. Performing the Fourier and PDM analysis in the frequency range from 0.5 to 10 c d-1 we did not find any significant period. On the other hand, a close look at the Hipparcos measurements reveals the presence of the one day increasing and one day fading of the brightness of the order of 0.[FORMULA] 05. The character of these variations is not clear because of the lack of longer series of observations.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999