  We study real numbers $\beta$ with the curious property that the
$\beta$-expansion of all sufficiently small positive rational numbers is purely
periodic. It is known that such real numbers have to be Pisot numbers which are
units of the number field they generate. We complete known results due to
Akiyama to characterize algebraic numbers of degree 3 that enjoy this property.
This extends results previously obtained in the case of degree 2 by Schmidt,
Hama and Imahashi. Let $\gamma(\beta)$ denote the supremum of the real numbers
$c$ in $(0,1)$ such that all positive rational numbers less than $c$ have a
purely periodic $\beta$-expansion. We prove that $\gamma(\beta)$ is irrational
for a class of cubic Pisot units that contains the smallest Pisot number
$\eta$. This result is motivated by the observation of Akiyama and Scheicher
that $\gamma(\eta)=0.666 666 666 086 ...$ is surprisingly close to 2/3.
