We propose a novel automata model over the alphabet of rational numbers, which we call register automata over the rationals (RA Q ). It reads a sequence of rational numbers and outputs another rational number. RA Q is an extension of the well-known register automata (RA) over infinite alphabets, which are finite automata equipped with a finite number of registers/variables for storing values. Like in the standard RA, the RA Q model allows both equality and ordering tests between values. It, moreover, allows to perform linear arithmetic between certain variables. The model is quite expressive: in addition to the standard RA, it also generalizes other well-known models such as affine programs and arithmetic circuits.The main feature of RA Q is that despite the use of linear arithmetic, the so-called invariant problem-a generalization of the standard non-emptiness problem-is decidable. We also investigate other natural decision problems, namely, commutativity, equivalence, and reachability. For deterministic RA Q , commutativity and equivalence are polynomial-time inter-reducible with the invariant problem. * Though normally called registers, for reasons that will be apparent later, we will refer to them as variables in this paper. 978-1-5090-3018-7/17/$31.00 c 2017 IEEE § A guard is non-strict if it does not contain negations, i.e., it is a positive Boolean combination of inequalities z z ′ .