We point out several problems with the standard definition
 of combination of universal theories when additional first-order
 axioms are presented. We give a new definition for the case
 of integrating first-order logic and rational arithmetic. After
 that we present a method of integrating linear rational arithmetic
 into the superposition calculus for first-order logic. We prove
 several incompleteness results and a completeness result under some
 finiteness assumptions.