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P1724 is a recently reported, X--ray active PMS star (Preibisch et al. 1995). Neuhäuser et al. (1995) found an X--ray variability with modulation on a 20 hour time scale in the star and speculated that the variability was the result of rotational modulation. P1724 was monitored from MTSB on 17 nights over a six week period. About five observations were made per night using V, R and I filters. Three field stars were used for comparison (see Figure 3). The results of these observations are shown in Figure 4. One can see from the figure the effects of aliasing. There is a family of several periods indicated by the periodogram. The preferred frequency is 1.18 rotations per day (a 0.85 day long period), but the power at this peak is not particularly stronger than the three other strong peaks. However, the 1.21 day period (0.826 rotations per day) is excluded by the nightly trend analysis. In this case, the trend of the data is opposite to the direction of the fit (see Figure 5). Evaluation of the 5.5 day period is more difficult. The dispersion of the data in any given night is only 10% greater than that of the 0.85 day solution. However, when the data are fit to the 5.5 day period, each given night has the mild trend, expected from the shorter period. Only one field standard star was available for the TAP 26 field. This, and the fact that the signal to noise is higher for P1724, are the reasons for the much cleaner results of the P1724 periodogram with respect to that of TAP 26.




Scott J. Wolk
Thu Dec 5 13:29:36 EST 1996