Relative X-ray flux curve of δ Cep . The observations in this plot were obtained over nearly 7 years. Although X-ray activity is present at all observed phases, there is a ∼4× increase in activity centered at ∼0.45ϕ. This is just after the star’s maximum diameter and prior to attaining minimum photospheric temperature (see Figure 1). The XMM-Newton data have been phased to the ephemeris used in Engle (2015), Engle et al. (2014): ﹩2455479.905+5.366208(14)\times E﹩. The Chandra data have been phased to the newly determined ephemeris: ﹩2456859.039+5.366279(21)\times E﹩. The close agreement in the phasing, flux levels, and behaviors of the data sets are the strongest evidence that the X-ray variations are tied to the pulsation period of the Cepheid.