BG Canis Minoris/3A 0729+103 : the true spin period revealed ?

A Ginga X-ray observation of the intermediate polar BG CMi is reported. It is demonstrated that the previously accepted 913-s spin period of the white dwarf in BG CMi must now be taken to represent the beat period of the system instead. The strong beat modulation provides clear evidence that the accretion in BG CMi does not occur predominantly via a disk and that this modulation arises as a result of the accretion stream flipping from one magnetic pole of the white dwarf to the other every half a beat cycle. The weak X-ray spin modulation may indicate that some accretion occurs via an accretion disk in addition to the predominant stream-fed accretion. The X-ray behavior of BG CMi is similar to that of TX Col in showing a stronger beat period modulation than spin period modulation, and in fact exhibits the highest ratio of power in X-ray beat modulation to spin modulation of any intermediate polar. It is suggested that the relative strengths of the spin and beat modulations seen in intermediate polars may reflect the relative importance of stream-fed and disk-fed accretion in each system.