Radial-velocity variations of the H \alpha emission measured on the steep wings of the H \alpha line , prewhitened for the long-time changes , vary periodically with a period of 218 \aas@@fstack { d } 025 \pm 0 \aas@@fstack { d } 022 , confirming the suspected binary nature of the bright Be star BU Tau , a member of the Pleiades cluster .
The orbit seems to have a high eccentricity over 0.7 , but we also briefly discuss the possibility that the true orbit is circular and that the eccentricity is spurious owing to the phase-dependent effects of the circumstellar matter .
The projected angular separation of the spectroscopic orbit is large enough to allow the detection of the binary with large optical interferometers , provided the magnitude difference primary - secondary is not too large .
Since our data cover the onset of a new shell phase up to development of a metallic shell spectrum , we also briefly discuss the recent long-term changes .
We confirm the formation of a new envelope , coexisting with the previous one , at the onset of the new shell phase .
We find that the full width at half maximum of the H \alpha profile has been decreasing with time for both envelopes .
In this connection , we briefly discuss Hirata ’ s hypothesis of precessing gaseous disk and possible alternative scenarios of the observed long-term changes .