Discovery of GRS 1915+105 variability patterns in the Rapid Burster

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We report the discovery of two new types of variability in the neutron star low-mass X-ray binary MXB 1730-335 (the 'Rapid Burster'). In one observation in 1999, it exhibits a large-amplitude quasi-periodic oscillation with a period of about 7 min. In another observation in 2008, it exhibits two 4-min long 75 per cent deep dips 44 min apart. These two kinds of variability are very similar to the so-called ρ or 'heartbeat' variability and the θ variability, respectively, seen in the black hole low-mass X-ray binaries GRS 1915+105 and IGR J17091-3624. This shows that these types of behavior are unrelated to a black hole nature of the accretor. Our findings also show that these kinds of behaviour need not take place at near-Eddington accretion rates. We speculate that they may rather be related to the presence of a relatively wide orbit with an orbital period in excess of a few days and about the relation between these instabilities and the type II bursts.