  For an arbitrary proper DG algebra A (i.e. DG algebra with finite dimensional
total cohomology) we introduce a pairing on the Hochschild homology of A and
present an explicit formula for a Chern-type character of an arbitrary perfect
A-module (the Chern characters take values in the Hochschild homology of A).
The Hirzebruch-Riemann-Roch formula in this context expresses the Euler
characteristic of the Hom-complex between two perfect A-modules in terms of the
pairing of their Chern characters. We mention two examples of proper DG
algebras and the HRR formulas for them. The first example is Ringel's formula
for quivers with relations. The second example is related to orbifold
singularities of the form V/G where V is a complex vector space and G is a
finite subgroup of SL(V). Furthermore, we prove that the above pairing on the
Hochschild homology is non-degenerate when the DG algebra is smooth. We also
formulate the conjecture that for a Calabi-Yau DG algebra A the pairing
coincides with the one coming from the Topological Field Theory associated with
A and verify it in the case of Frobenius algebras.
