Over the past decade , the Secret Lives of Cepheids ( SLiC ) program has been carried out at Villanova University to study aspects and behaviors of classical Cepheids that are still not well-understood .
In this , the first of several planned papers on program Cepheids , we report the current results for ( catalog $ δ $ Cep ) , the Cepheid prototype .
Ongoing photometry has been obtained to search for changes in the pulsation period , light curve morphology and amplitude .
Combining our photometry with the times of maximum light compilation by Berdnikov et al .
( 11 ) returns a small period change of dP/dt \approx - 0.1006 \pm 0.0002 sec yr ^ { -1 } .
There is also evidence for a gradual light amplitude increase of \sim 0.011-mag ( V -band ) and \sim 0.012-mag ( B -band ) per decade over the last \sim 50 years .
In addition , HST-COS UV spectrophotometry and XMM-Newton X-ray data were carried out to investigate the high-temperature plasmas present above the Cepheid photospheres .
In total , from the five visits ( eight exposures ) with XMM-Newton , ( catalog $ δ $ Cep ) is found to be a soft X-ray source ( L _ { X } [ 0.3–2 keV ] \approx 4.5 - 13 \times 10 ^ { 28 } erg sec ^ { -1 } ) with peak flux at kT = 0.6–0.9 keV .
The X-ray activity is found to vary , possibly in phase with the stellar pulsations .
From 2010–2013 , nine observations of ( catalog $ δ $ Cep ) were carried out with HST-COS .
The UV emissions are also variable , and well-phased with the stellar pulsations .
Maximum UV line emissions occur near , or slightly before , maximum optical light , varying by as much as twenty times .
This variability shows that pulsation-induced shock-heating plays a significant role in Cepheid atmospheres , possibly in addition to a quiescent , magnetic heating .
The results of this study show Cepheid atmospheres to be rather complex and dynamic .