1 Introduction

In close binary systems light curves, radial velocity curves and line profiles are the most important sources of information to find the physical parameters of the components. The atmosphere of the component of a close binary system are distorted by the tidal effect due to the presence of its companion, and self rotation. Nonuniform temperature distribution over its surface (which may arise due to gravity darkening), and the additional heating of the stellar surface by incident radiation from the companion star (particularly important for X-ray binaries), eclipses, stellar wind, and interaction effects associated with circumstellar gas structures (disks, jets, shells etc.) will change the radiation field in the atmosphere.

Theoretical studies about the reflection effect using actual model atmospheres are more recent. All the three possible combinations (i.e., like when primary and the secondary components are hot and hot, cool and cool, and hot and cool) are studied by Buerger (1969, 1972), Nordlund & Vaz (1990), Claret & Gemenz (1992) respectively. Vaz (1985) & Wilson (1990) reviewed several aspects of reflection effect. They found that irradiation from the secondary component will effect the lines and as well as equivalent widths. They also found that theoretical bolometric albedos have been found to be in good agreement with observations. Peraiah & Srinivasa Rao (1983) studied effects of reflection on formation of spectral lines in a purely scattering atmosphere and how the equivalent widths change due to irradiation from the secondary. However these calculations were done in static atmospheres. Recently Peraiah & Srinivasa Rao (1998, hereafter referred to as Paper I) have studied this problem of irradiation on line formation in an expanding atmosphere of the component of a close binary system and P-Cygni type profiles are found to be produced. The study of line formation in the dusty envelopes remains practically unexplored. The presence of dust is revealed in the infrared observations of many stellar objects, like gaseous nebulae, active galactic nuclei, T-Tauri stars and binary stars. Presence of dust, radial expansion, geometrical extension, chemical composition etc. are some of the physical and geometrical properties one should include in any serious calculation of line formation in such atmospheres. The purpose of this study is to compute the effects of dust on the formation of lines in the expanding atmospheres with light of the secondary falling on the component in a close binary system. We consider that the dust scatters radiation isotropically and neither dust absorption nor emission are taken into account.