The Mira variable R Hydrae is well known for its declining period , which Wood & Zarro ( 1981 ) attributed to a possible recent thermal pulse .
Here we investigate the long-term period evolution , covering 340 years , going back to its discovery in AD 1662 .
The data includes photometric monitoring by amateur and other astronomers over the last century , and recorded dates of maximum for earlier times .
Wavelets are used to determine both the period and semi-amplitude .
We show that the period decreased linearly between 1770 and 1950 ; since 1950 the period has stabilized at 385 days .
The semi-amplitude is shown to closely follow the period evolution .
Analysis of the oldest data shows that before 1770 the period was about 495 days .
We find no evidence for an increasing period during this time as found by Wood & Zarro .
We discuss the mass-loss history of R Hya : the IRAS data shows that the mass loss dropped dramatically around AD 1750 .
The evolution of the mass loss as function of period agrees with the mass-loss formalism from Vassiliadis & Wood ; it is much larger than predicted by the Blöcker law .
An outer detached IRAS shell suggests that R Hya has experienced mass-loss interruptions before .
The period evolution can be explained by two models : a thermal pulse occuring around AD 1600 , or an non-linear instability leading to an internal relaxation of the stellar structure .
The elapsed time between the mass-loss decline giving rise to the outer detached shell , and the recent event , of approximately 5000 yr suggests that only one of these events could be due to a thermal pulse .
Further monitoring of R Hya is recommended , as both models make strong predictions for the future period evolution .
We argue that R Hya-type events could provide part of the explanation for the rings seen around some AGB and post-AGB stars .
Changes in Mira properties were already known on a cycle-to-cycle basis , and on the thermal-pulse time scale of \sim 10 ^ { 4 } yr .
R Hya shows that significant evolution can also occur on intermediate time scales of order 10 ^ { 2 } – 10 ^ { 3 } yr .