  A general dilaton gravity theory in 1+1 spacetime dimensions with a
cosmological constant $\lambda$ and a new dimensionless parameter $\omega$,
contains as special cases the constant curvature theory of Teitelboim and
Jackiw, the theory equivalent to vacuum planar General Relativity, the first
order string theory, and a two-dimensional purely geometrical theory. The
equations of this general two-dimensional theory admit several different black
holes with various types of singularities. The singularities can be spacelike,
timelike or null, and there are even cases without singularities. Evaluation of
the ADM mass, as a charge density integral, is possible in some situations, by
carefully subtrating the black hole solution from the corresponding linear
dilaton at infinity.
