COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2165 Konkoly Observatory Budapest 1982 June 17 HU ISSN 0374-0676 THE PRIMARY ECLIPSE OF RW MONOCEROTIS IN FIVE COLORS RW Monocerotis has long been recognized as an Algol-type eclipsing binary with a deep primary minimum due to a complete occultation. Szczpanowska (1951) discusses the extensive series of visual observations made at the Cracow Observatory between 1924 and 1949. Batten (1956, 1957) obtained orbital elements for this system using visual observations made by Dugan and Pierce and photoelectric observations made by Lenouvel. Brukalska et al. (1969) obtained an extensive series of photoelectric observations of this system in the red (Lambda_eff ca. 6800 A) and in the infrared (Lambda_eff - ca. 8000 A). Orbital elements were obtained by Rucinski (1970) using these observations. These data have been subsequently analyzed by Mezzetti et al. (1980) using the Wood method. In this investigation the primary eclipse of RW Mon was observed in five colors on two nights in January, 1982, with the 1.0 meter Ritchey-Chretien telescope of the Flagstaff Station of the U.S. Naval Observatory. The photomultiplier used was an EMI 9658R (S-20 surface) refrigerated by means of a thermoelectric cooler. The photometric system used closely resembles the UBVRI system developed by Cousins (1973, 1976). The effective wavelengths of the bandpasses are as follows: U 3550 A B 4350 V 5450 R 6150 I 8100 The star BD +08d 1400 was used as a comparison. The colors and magnitudes obtained for this star were V = 9.53, B - V = +0.02, U - B = -0.07, V - R = +0.02 and. V - I = +0.04. A total of 355 observations of RW Mon was obtained. These observations, corrected for light time and differential extinction, have been transformed to the UBVRI system of Cousins. They have been placed in the archives of the Royal Astronomical Society. The magnitude differences listed are in the sense RW Mon - BD +08d1400. The ranges of the magnitudes and depths of the primary minimum of RW Mon are as follows: U max.= -0.42 min.= +3.05 depth (I) = 3.47m B -0.24 +2.68 2.92 V -0.24 +1.98 2.22 R -0.27 +1.55 1.82 I -0.31 +1.20 1.51 [FIGURE 1] In Figure 1 in which the observations are plotted, each marking on the vertical scale represents 0.50 mag. The I passband used in this investigation closely resembles the one (series I) used by Brukalska et al., but their passband in the red (series II) has a much longer effective wavelength than the R band used here. The magnitudes and colors obtained for RW Mon (HDE 259986), whose spectral type is given as A0, are as follows: V at maximum = 9.29 at primary = 11.51 B - V +0.02 +0.72 V - B -0.26 +0.30 V - R +0.01 +0.42 V - I +0.11 +0.75 At maximum light the colors of this system are most similar to those of a B9 star, while at minimum they closely resemble those of a G5 star. RW Mon does not appear to be significantly reddened. Thus we can regard RW Mon as consisting of components whose spectral types are B9 V and G5 IV. The duration of the total phase portion of the eclipse was found by this investigator to be 0.051d or 1h13m. This corresponds to a phase angle of internal tangency of 4.82d, a value which is somewhat larger than that reported by previous investigators. For the phase angle of external tangency, however, he found a value of 27.4d, which is significantly less than that reported by previous observers of RW Mon. One time of minimum light was found in this investigation, JD Hel. 2444979.7733. When this is combined with the other photoelectric times of minimum light the following data are obtained: JD 2433680.4491 E = 0 O-C = +0.0010d Batten 8443.77695 2499 -0.0004 Brukalska et al. 8445.68318 2500 -0.0002 " 9455.91152 3030 -0.0018 " 2443864.7101 5343 +0.0011 Olson 3883.7710 5353 +0.0011 " 4979.7733 5928 -0.0008 Chambliss Using a least-squares solution in which each of these times is given unit weight the following ephemeris is obtained: JD Hel. 2433680.4481 + 1.90609412d E. +- 6 +- 16 p. e. The residuals listed above are those obtained using this ephemeris. The period of RW Mon appears to have remained constant for the past 30 years. It should be noted, however, that this period is 0.6s shorter than that reported by Szczpanowska for observations made up to 1949. This investigator wishes to acknowledge the support which he has received from a Small Research Grant awarded by the American Astronomical Society. He also wishes to express his thanks to Drs. Harold D. Ables and Frederick J. Vrba of the U. S. Naval Observatory, Flagstaff, Arizona, for making facilities available to him at that institution. CARLSON R. CHAMBLISS Dept. of Physical Sciences Kutztown State College Kutztown, Penna., USA References: Batten, A. H. 1956, M.N.R.A.S., 116, 552 [BIBCODE 1956MNRAS.116..552B ] Batten, A. H. 1957, Ann. d'Ap., 20, 103 [BIBCODE 1957AnAp...20..103B ] Brukalska, R., Rucinski, S. M., Smak, J., and Stepien, K. 1969, Acta Astron., 19, 257 [BIBCODE 1969AcA....19..257B ] Cousins, A. W. J. 1973, Mem R. A. S., 77, 223 [BIBCODE 1973MmRAS..77..223C ] Cousins, A. W. J. 1976, Mem.R. A. S., 81, 25 [BIBCODE 1976MmRAS..81...25C ] Mezzetti, M., Cester, B., Giuricin, G., and Mardirossian, F. 1980, Astron. Astrophys. Suppl. Ser., 39, 273 [BIBCODE 1980A&AS...39..273M ] Olson, E. C. 1980, I.B.V.S. No. 1840 Rucinski, S. M. 1970, Acta Astron., 20, 351 [BIBCODE 1970AcA....20..351R ] Szczpanowska, A. 1951, Acta Astron., Series C, 4, 137