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Circumstellar shells around AGB stars are built over long periods of timethat may reach several million years. They may therefore be extended over largesizes (~1 pc, possibly more), and different complementary tracers are needed todescribe their global properties. In the present work, we combined 21-cm HI andCO rotational line data obtained on an oxygen-rich semi-regular variable, RXLep, to describe the global properties of its circumstellar environment. Withthe SEST, we detected the CO(2-1) rotational line from RX Lep. The line profileis parabolic and implies an expansion velocity of ~4.2 km/s and a mass-lossrate ~1.7 10^-7 Msun/yr (d = 137 pc). The HI line at 21 cm was detected withthe Nancay Radiotelescope on the star position and at several offset positions.The linear shell size is relatively small, ~0.1 pc, but we detect a trailextending southward to ~0.5 pc. The line profiles are approximately Gaussianwith an FWHM ~3.8 km/s and interpreted with a model developed for the detachedshell around the carbon-rich AGB star Y CVn. Our HI spectra are well-reproducedby assuming a constant outflow (Mloss = 1.65 10^-7 Msun/yr) of ~4 10^4 yearsduration, which has been slowed down by the external medium. The spatial offsetof the HI source is consistent with the northward direction of the propermotion, lending support to the presence of a trail resulting from the motion ofthe source through the ISM, as already suggested for Mira, RS Cnc, and othersources detected in HI. The source was also observed in SiO (3 mm) and OH (18cm), but not detected. The properties of the external parts of circumstellarshells around AGB stars should be dominated by the interaction between stellaroutflows and external matter for oxygen-rich, as well as for carbon-rich,sources, and the 21-cm HI line provides a very useful tracer of these regions.

HI and CO in the circumstellar environment of the oxygen-rich AGB star RX Lep