COMMISSIONS 27 AND 42 OF THE IAU
               INFORMATION BULLETIN ON VARIABLE STARS
                            Number 4044


                                                    Konkoly Observatory
                                                    Budapest
                                                    1 July 1994

                                                    HU ISSN 0374 - 0676


              VARIATIONS OF THE Ap STAR IN NGC 2169  ^1


  A period of 1.56d had been determined by Maitzen & Lebzelter (1993) for the Delta alpha and
uvby variations of the Ap star Rns11180 (Renson 1991) = NGC2169-9 (Cuffey & 
McCuskey 1956) = NGC2169-12 (Hoag et al. 1961). This value has been recently questioned
by Renson (1994), who favors a period of 0.606d for the same data. However, these periods
are based on only 7 Delta alpha observations (Maitzen & Lebzelter 1993), and on two series of
uvby data (Delgado et al. 1992) covering part of two nights.
  Fortunately, four series of uvby CCD frames of the cluster had been secured during a
four-night interval in January 94, at the 91cm Dutch telescope of the La Silla observatory.
  The amplitude of the variations decreases from about 0.15m in u down to about 0.06m in
y, making the ultraviolet and blue channels the most interesting for the period analysis.
Figures 1 and 2 show the periodograms in, respectively, u and v.
  Analysis of these data yields the following possible frequencies (periods):AA
`
  f_1 = 0.321 d^-1 (P_1 = 3.111d)
  f_2 = 0.642 d^-1 (P_2 = 1.557d)
  f_3 = 0.797 d^-1 (P_3 = 1.254d)
  f_4 = 1.618 d^-1 (P_4 = 0.618d)

  Examination of the lightcurves indicates that the most probable is the second one, in
agreement with Maitzen & Lebzelter's (1993) results.
  It appears that f_1 = f_2/2 corresponds to a period twice the favored period (P_2);
f_3 ~ f_4/2; and f_4  ~ 1+f_2
  Hence f_4, which had been proposed by Renson (1994), simply is a 1-day alias of f_2.
  By combining the 1994 data with those of Delgado et al. (1992), using the method
described by Manfroid & Renson (1994), we end up with a large series of possible values:

          f = 0.639795 +/- 0.000012 + n x 0.0003364 d^-1

with n<= 10. This corresponds to P = 1.56300d +/- 0.00001d and aliases.
  The v periodogram around frequency f_2 is shown in Fig. 3, while Figs. 4 and 5 show
the v and u complete light curves folded with P = 1.56300d. As a comparison, the v light
curve has been drawn in Fig. 6 for the best period found around 0.606d (namely 0.61861d).
  While our results do not unambiguously solve the problem of the period of NGC 2169-
9, they point toward a value close to the one proposed by Maitzen & Lebzelter (1993)
instead of the much shorter one favored by Renson (1994).


                               [FIGURE 1]

  Figure 1: Periodogram in the u band for the 1994 data. The ordinate is the inverse 
  of the chi-square calculated in a two-sine fit (see Manfroid and Renson 1994).

                               [FIGURE 2]

  Figure 2: Periodogram in the v band for the 1994 data. 

                               [FIGURE 3]

  Figure 3: Periodogram around f_2 in the v band for the 1994 data and Delgado
  et al.'s (1992) data.

                               [FIGURE 4]

  Figure 4: Lightcurve in the v band for the 1994 data (crosses) and Delgado
  et al.'s data (squares). P=1.56300d and the phase origin is JD 2449279.0 . 
  Ticks on the ordinate axis are separated by 0.01m.

                               [FIGURE 5]

  Figure 5: Same as Fig. 4 for the u band.

                               [FIGURE 6]

  Figure 6: Lightcurve in the v band for the 1994 data and Delgado et al.'s
  data. P=0.61861 and the phase origin is JD 2449279.0




 ^1 Based on observations carried out at the ESO La Silla Observatory



                              J. MANFROID
                              P. RENSON
                              Institut d'Astrophysique
                              Avenue de Cointe 5
                              B-4000 Liege, Belgium


References:

Cuffey J., McCuskey S.W.: 1956, ApJ 123, 59 [BIBCODE 1956ApJ...123...59C ]
Delgado A.J., Alfaro E.J., Garcia-Pelayo J.M.: 1992, AJ 103, 891 [BIBCODE 1992AJ....103..891D ]
Hoag A.A., Johnson H.L., Iriarte B., Mitchell R.I., Hallam K.L., Sharpless S.: 1961, Publ.
  U.S. Naval Obs. 2nd Ser., 7, 345 [BIBCODE 1961PUSNO..17....1H ]
Maitzen H.M., Lebzelter Th.: 1993, IBVS 3856
Manfroid J., Renson P.: 1994, A&A, 281, 73 [BIBCODE 1994A&A...281...73M ]
Renson P.: 1991, Catalogue General des Etoiles Ap et Am, Institut d'Astrophysique,
  Universite de Liege [BIBCODE 1991cgde.book.....R ]
Renson P.: 1994, A&A, 285, 912 [BIBCODE 1994A&A...285..912R ]