Photometric study of new southern SU UMa-type dwarf novae and candidates – II. Authentication of BF Ara as a normal SU UMa-type dwarf nova with the shortest supercycle

We photometrically observed the 2002 August long outburst of BF Ara. The observation for the first time unambiguously detected superhumps [average period 0.087 97(1) d], qualifying BF Ara as a genuine SU UMa-type dwarf nova. An analysis of the long-term visual light curve yielded a mean supercycle length of 84.3(3) d. The characteristics of outbursts and superhumps more resemble those of usual SU UMa-type dwarf novae rather than those of ER UMa stars. BF Ara is thus confirmed to be a usual SU UMa-type dwarf nova with the shortest known supercycle length. There still remains an unfilled gap of distributions between ER UMa stars and the usual SU UMa-type dwarf novae. We detected a zero period change of the superhumps, which is quite unexpected from our previous knowledge. This discovery implies that a previous interpretation requiring a low would no longer be valid or that a different mechanism is responsible for BF Ara. We propose that the reduced (prograde) apsidal motion of the eccentric disc by pressure forces may be responsible for the unusual period change in BF Ara.