H.P. Williams states in his book "Model Building in Mathematical Programming:
"An SOS1 is a set of variables (continuous or integer) within which exactly one variable must be non-zero."
So, if I have two binary variables a, b which also form a SOS1 then the SOS1 property together with the constraint a + b = 1 impose integrality on a, b and therefore I don't need to require explicitely the integrality of a,b.
CPLEX defines a SOS1 as:
"SOS Type 1 is a set of variables where at most one variable may be nonzero."
Is there a way to avoid explicitely requiring the integrality of a,b as in the case of Williams?