Rigidity times for weakly mixing dynamical system which are not rigidity   times for any irrational rotation
  We construct an increasing sequence of natural numbers $(m_n)_{n=1}^{+\infty}$ with the property that $(m_n \th [1])_{n\geq 1}$ is dense in $\T$ for any $\th \in \R\setminus \Q$, and a continuous measure on the circle $\mu$ such that $\lim_{n\to +\infty}\int_{\T}\|m_n\theta\|d\mu(\theta)=0$. Moreover, for every fixed $k\in \N$, the set $\{n\in \N:\,k\nmid m_n \}$ is infinite.   This is a sufficient condition for the existence of a rigid, weakly mixing dynamical system whose rigidity time is not a rigidity time for any system with a discrete part in its spectrum. 
Bassam Fayad and Adam Kanigowski
math.DS