Implication of Existence of Hybrid stars and Theoretical Expectation of   Submillisecond Pulsars
  We derive the bulk viscous damping timescale of hybrid stars, neutron stars with quark matter core. The r-mode instability windows of the stars show that the theoretical results are consistent with the rapid rotation pulsar data, which may give an indication for the existence of quark matter in the interior of neutron stars. Hybrid stars instead of neutron or strange stars may lead to submillisecond pulsars. 
Xiaoping Zheng, Nana Pan, Shuhua Yang, Xuewen Liu, Miao Kang, Jiarong   Li