Recently we have started a systematic reevaluation of the existing observational methods of analysis hitherto applied to PCB-s. Here we report the first results of our investigations. The improved Napier's algorithm (Napier, 1968) to model the light curves of PCB (with the aid of the set of our computer programs in Turbo Pascal) is used. The source function is taken either from Sobieski (1965) or Strittmatter (1974). The entire luminosity received from the cool companion is calculated by integration of the emerging radiation over its disk; contributions from the illuminated and unilluminated portions of the disk are accounted for. We assume: a) the validity of the LTE in the photosphere of cool component, b) constancy of the monochromatic to mean absorption coefficient ratio within the photosphere, c) the hot star radiates as the absolutely black body, d) there are no other effects influencing the light curve except for the reflection effect. We have modelled the light curves of EC11575-1845 (Chen et al (1995). The analysis of the temperature distributions in the heated photosphere indicates the occurence of the temperature inversion. To make a rough estimate of physical conditions which can induce generation of evaporative wind we have used two criteria: i) the temperature inversion, ii) relation between radiative pressure and the effective gravitational acceleration. We assumed the density varies with the height in the atmosphere of the illuminated star as ρ ≃ exp(−Φ/RgT), Φ - being the Roche potential. The integral equation following from the definition of the mean optical depth was solved numerically to establish the relation between the mean optical depth and the distance in the atmosphere. We find the characteristic height scale for X-ray and EUV radiation is ∼ 106 − 107cm (for concentration of particles ∼ 1013cm−3). To check the validity of the ii) criterium we used a simplified model of radially expanding evaporative stellar wind and mass flux J conservation condition along the stream tube of the form J ∼ ρsvsrs2 exp −(Φ/RgT) (Pustylnik (1995)) and found M ∼ 10−12–10−11M/y. Although mass loss of such a rate cannot compete with the effects of the angular momentum loss which is responsible for a secular orbital shrinkage, the evaporative wind should significantly alter the structure of the the cool irradiated components.

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Between 1969 and 1993 a number of UBVR light curves of the long-period W Ser-type interacting binary RX Cas (P orb = 32.328 days) were obtained at Tallinn Observatory. The results of earlier observations, the interpretation of the light curves as well as the investigation of the nature of invisible primary component, intrinsic variability and its period (P intr = 516.06 days) were summarized by Kalv (and also discussed by Martynov). In recent years, new solutions of the radial velocity curves as well as of the synthetic light curves of RX Cas have been obtained. This enables the physical parameters of the components to be determined more accurately and the rate of mass exchange between the components and the initial parameters of the progenitor to be estimated with better precision.RX Cas belongs to a small group of W Ser-type binaries which according to our current knowledge are caught in a state of high-rate mass loss and mass transfer. The other beststudied members of the W Ser group are SX Cas, W Crucis, V367 Cygni, β Lyrae and W Ser itself. A common feature for this group is the presence of rich ultraviolet emission line spectra coming from higher ionization levels than can be expected from the effective temperatures of the stellar components.The early stage of studies of RX Cas was summarized by Dmitri Yakovlevich Martynov [1, 2] who was the first to propose the quantitative model of RX Cas and to study in detail the effect of apsidal motion in this system, estimating the total period of apsidal motion as = 29 000 ± 4 000 years. Kalv [3] found a reliable period P = 516.06 days of the intrinsic variability in RX Cas. Additional UBVR light curves of RX Cas were obtained at Tallinn Observatory in the observational period following the publication by Kalv [3] of the comprehensive analysis of the data consisting of the earlier observations obtained between 1969 and 1979 (see also [4,5]). New R light curves [6] are shown in figure 1. The orbital phases were calculated with a value for the orbital period of P = 32.327 39 days. In figure 2, the O − C diagram is shown covering the total period of photoelectric photometry of RX Cas at Tallinn Observatory. Note that our orbital period is slightly longer than the value of P = 32.3238 days given by Kreiner et al. [7] but the trend (secular lengthening of the period) is the same. From the value dP /P ≈ 10 −7