  Let $G$ be a group acting on a set $X$ of combinatorial objects, with finite
orbits, and consider a statistic $\xi : X \to \mathbb{C}$. Propp and Roby
defined the triple $(X, G, \xi)$ to be \emph{homomesic} if for any orbits
$\mathcal{O}_1, \mathcal{O}_2$, the average value of the statistic $\xi$ is the
same, that is \[\frac{1}{{|\mathcal{O}_1|}}\sum_{x \in \mathcal{O}_1} \xi(x) =
\frac{1}{|\mathcal{O}_2|}\sum_{y \in \mathcal{O}_2} \xi(y).\]
  In 2013 Propp and Roby conjectured the following instance of homomesy. Let
$\mathrm{SSYT}_k(m \times n)$ denote the set of semistandard Young tableaux of
shape $m \times n$ with entries bounded by $k$. Let $S$ be any set of boxes in
the $m \times n$ rectangle fixed under $180^\circ$ rotation. For $T \in
\mathrm{SSYT}_k(m \times n)$, define $\sigma_S(T)$ to be the sum of the entries
of $T$ in the boxes of $S$. Let $\langle \mathcal{P} \rangle$ be a cyclic group
of order $k$ where $\mathcal{P}$ acts on $\mathrm{SSYT}_k(m \times n)$ by
promotion. Then $(\mathrm{SSYT}_k(m \times n), \langle \mathcal{P} \rangle,
\sigma_S)$ is homomesic.
  We prove this conjecture, as well as a generalization to cominuscule posets.
We also discuss analogous questions for tableaux with strictly increasing rows
and columns under the K-promotion of Thomas and Yong, and prove limited results
in that direction.
