  We investigate the spin-1/2 Heisenberg model on the delta chain (sawtooth
chain) with ferromagnetic nearest-neighbor and antiferromagnetic next-neighbor
interactions. For a special ratio between these interactions there is a class
of exact ground states formed by localized magnons and the ground state is
macroscopically degenerate with a large residual entropy per spin
$s_0=\frac{1}{2}\ln 2$. An important feature of this model is a sharp decrease
of the gaps for excited states with an increase of the number of magnons. These
excitations give an essential contribution to the low-temperature
thermodynamics. The behavior of the considered model is compared with that of
the delta chain with both antiferromagnetic interactions.
