對光學天文觀測而言，目前已經進到了巡天計畫的時代。從早期的帕洛瑪照相底片巡天計畫，到近期如 SDSS, PTF, CRTS 等等的巡天計畫。這些巡天計畫因為具有廣視野，所以對於某類天體的系統性研究有非常好的助益。有幸的是中央大學在 2006 年及 2009 年分別加入了兩個大型的國際巡天計畫 PS1 及 PTF。這兩個計畫都是由許多國家共同參與，各自提供有興趣的科學目標來進行研究。而激變變星的光變研究是我們感興趣的科學題目。而我們預期 PTF 計畫對於較長周期的訊號會有較佳的偵測結果。所以我們著手進行的激變變星的長周期性研究。由於我們有 PTF 觀測處理之後每>顆偵測到的星體的光度資料。我們便建立了一個小型但迅速的系統來快速地獲得光變曲線。這個系統幫助我們在龐大的資料中找到我們有興趣的天體的光變曲線。

我們利用了 Downes 所作的已知激變變星星表作為參考，在 PTF 資料庫中找尋相對應的激變變星。在星表中的 1830 個星體中，我們找到了 344 個激變變星。排除掉了比較少觀測的星體後，約有一百個激變變星具有足夠的資料點可以進行時間序列分析。我們利用了 Lomb-Scargle 及 PDM 等尋找周期的方法，對於這些光變曲線進行分析。最後找到了十個具有長周期性訊號的激變變星。這些長周性性的訊號，有可能是由不同機制造成的。所以我們針對這些可能的機制進行了探討。這>些機制主要有可能是吸積盤的進動、三星系統、伴星的磁場變化、以及一些其他可能的機制等。在我們發現的這些目標中，主要的長周期成因可能為三星系統。而有少數較短的周期可能是由於吸積>盤的進動所造成。至於其他可能的機制，我們初步排除了可能性。這個研究是為了未來超大型巡天的先導研究。利用相似的方法，當超大型巡天上線之後，我們就可以立即地增加我們目標的偵測率>，而對激變變星的長周期光變現象有更深入的了解。

It is the synoptic survey era for the astronomical optical observation. The evolution of the synoptic survey is from the formaer Palomar Sky Survey (PSS), to more recently SDSS, PTF, and CRTS surveys, etc. The major feature of synoptic surveys is wide field-of-view. Therefore, the systematic study on specific astronomical object is the niche of synoptic survey. Luckily, the National Central University joined two large synoptic survey in 2006 for Pan-STARRS1, and 2009 for PTF. These two projects are international cooperation, and many projects were proposed for the potential scientific studies with these surveys. What we are interested in is the variability of cataclysmic variables (CVs). We expected that the long-term variability of CVs are good to study with the PTF project. Therefore, we started the study of long-term variabilities of CVs.

We adopted Downes' catalog of CVs as a reference catalog for our study. A small but quick responsive data retrival system was established by us. We retrieved the light curves of CVs from this system. There are 344 CVs were matched in the PTF database. After filtering out the light curves with a few obsrvations, there are about 100 CVs were selected for further timing analysis. We adopt the Lomb-Scargle and PDM methods to analysis the light curves. Finally, there are 10 CVs found to possess long-term periodic signals. The lnog-term periodicities are possibly caused by different mechanisms. They are possibly from the precession of the accretion disk, hierarchical triple star system, the magnetic filed strength of the companion changes, and other possible mechanisms. We discussed the sources case by case, and concluded that the majority of the mechanism is possibly from the hierarchical triple system. On the other hand, a few sources with shorter long-term periodicities maybe cause by the precesion of the accretion disk. Basically, we could eliminate other possiblity on our CVs with the current evidence. This study is the pilot study on the long-term variability of the CVs with even larger synoptic surveys. When larger synotpic surveys are online, the method for the study may yield more results and help us to clearify the mechanisms of the long-term variabilitys of CVs.

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