2000MNRAS.312L..43P

Observations of δ Ori A made with the UHRF in its highest resolution mode (R~900000) have revealed the presence of a cool (T_k≤350K) variable absorption component at a heliocentric velocity of +21.3km.s^–1. The component is detected in Nai D₁, where clear hyperfine splitting is seen, and Caii K. Comparison of our data with existing spectra suggests that the component has consistently increased in strength from 1966 to 1994, and subsequently reduced in intensity by 1999. Following a discussion of the possible origins of this component it is concluded that an interstellar, rather than circumstellar, origin is most likely. This is one of very few detections of variable interstellar absorption reported in the literature, and we suggest an origin within filamentary material associated with the expanding Hi shell surrounding the Orion-Eridanus superbubble.