  We study the tau function of the KP-hierarchy associated with an (n,1) curve
$y^n=x-\alpha$. If $\alpha=0$ the corresponding tau function is 1. On the other
hand if $\alpha\neq 0$ the tau function becomes the exponential of a quadratic
function of the time variables. By applying vertex opertaors to the latter we
obtain soliton solutions on non-zero constant backgrounds.
