Former top politicians usually stay out of the limelight after they retire, but several former senior officials have shown up in public in recent weeks.

Analysts have suggested the appearances may be a way to voice support for the current state leadership or to offer comfort to former allies, some of whom were said to be targets of President Xi Jinping's anti-corruption campaign. Zeng and the senior Jiang are close allies.

Zeng appeared to be in good spirits and stayed for about an hour at the museum, according to staff. He is reportedly a friend of Han Tianheng, a painter and calligrapher, and they were said to have talked about Chinese antiquities.

The director of the museum, Zhu Xiaodong , declined to give details of the trip. "It was a private visit," Zhu said. Photographs of the occasion were later posted online.

In his anti-graft campaign, Xi has vowed to go after both the "tigers" and the "flies", meaning that both low- and high-level cadres would come under scrutiny.

The Communist Party has yet to make any announcement over the fate of retired national security chief Zhou Yongkang , who has been detained since December as part of a corruption investigation. If charged with graft, he would be the most senior official to face such accusations in recent decades.

Zeng is believed to have become acquainted with Zhou while the former security tsar was working in the liaison office at the China National Offshore Oil Corporation in the 1980s. But their relationship cooled after Zeng retired in 2008.

Jiang became mayor of Shanghai in 1985, and was later made its party chief. He became general secretary of the Communist Party after Zhao Ziyang was purged in 1989 following the Tiananmen Square protests.

Jiang later brought some of his former aides from Shanghai to Beijing, including Zeng.

Jiang was seen in public in Jiangsu province and in Shanghai in April. His successor, Hu Jintao, appeared at a university in Hunan province the same month.

Jiang's premier, Zhu Rongji, recently wrote a letter for the 30th anniversary of Tsinghua University's school of economics, where he served as dean.

Zhu's successor, Wen Jiabao, last week wrote a poem to a respected professor in his hometown of Tianjin .

Former top legislator Wu Bangguo and former propaganda tsar Li Changchun have also appeared at different locations on sightseeing visits. Wang Zhaoguo, a former member of the Politburo, also appeared in Jiangsu province in April with his wife.

The visit took place amid rumours his son may have been involved in a graft investigation.