  Electron tunneling through a system formed by two coupled quantum dots in a
parallel geometry is considered within a generalized Anderson model. The dots
are assumed to have nearly equal radii but different (and tunable) gate
voltages. In the absence of tunneling to and from the leads, the isolated
two-dot system (with two electrons in it) resembles an hydrogen molecule within
the Heitler London approximation. In particular, it has a singlet ground state
and low lying excited triplet state. When tunneling is switched on, and when
the gate voltages are properly tuned the ground state becomes a triplet and
singlet and triplet states are intermixed. In the region where charge
fluctuations are suppressed, the pertinent antiferromagnectic exchange
interaction has the form $(J^{T} {\bf S} +J^{ST} {\bf P}) \cdot {\bf s}$. It is
written in terms of the electron spin ${\bf s}$, the double dot spin 1 operator
${\bf S}$ and an additional vector operator ${\bf P}$. The operators ${\bf S}$
and ${\bf P}$ generate the algebra $o_{4}$ of a spin rotator. The related Kondo
effect is similar to that of a vertical quantum dot, discussed and analyzed
recently.
