1. Shell stars, defined as those having a deep absorption core bordered by emissions in Ha and other lines, arising from transitions between low or metastable levels. We distinguish two types of shell stars: (a) those that have a fairly constant radial velocity, with changes in the total intensity of the emission for example, 28 Tauri; and (b) those that show longperiod variations in radial velocity to which the relative intensities of the red and violet emission components are related for example, 27 Canis Majoris. The latter are known as V/R variables.