Zhang and Zhao, a couple in their 70s from Shenyang, Northeast China's Liaoning province, have lived happily together for some years, but they are not married -- both their families oppose such a marriage.

Zhang, in her late 70s, has bad eyes and legs. Each time she falls ill, Zhao, five years her junior, cares for her. On her ailing legs, whenever Zhang wants to go outside, Zhao accompanies her, serving as her eyes and a walking stick-like support.

"He is very nice to me," Zhang said with a smile on her face. "He cooks for me. He is a great cook."

The families of the two, however, say the wide gap in social status between the two -- Zhang is a retired cadre while Zhao is a laid-off worker -- is too great.

Though agreeing to their cohabitation, Zhang's sons strongly oppose a marriage, as they fear the issue of "property division" may become complicated.

An increasing number of such de facto relationships are occurring among elderly Chinese.

Xu Yujiang, in his late 80s, was living with a 50-year-old woman who was previously his nurse. Their marriage was resisted by Xu's children, who feared the nurse wanted to marry Xu to inherit his assets when he died.

Xu's identity card and household registration book -- both necessary documents to register marriage -- were even "stolen" by his children.

Prof. Li Jing from the Law School at Beijing Foreign Studies University suggests old couples and their families turn to mediation committees to persuade family members to respect their parent's wishes.

The Office of the China National Committee on Ageing said the number of people aged 60 or over in China in 2009 stood at 167 million, 12.5 percent of China's population of 1.3 billion.

A survey by the Chinese Academy of Social Sciences, a key government think-tank, shows 80 percent of widowed senior citizens want to remarry but that most choose to cohabit with rather than marry their partner. Less than 10 percent register a new marriage.

Sociologist Li Yinhe said that in an aging society, elderly remarrying has become increasingly common.

"Property-division is at the center of the problem. Couples sometimes choose to delay marriage as they are not certain about their partner," Li said.

She added that public pressures is also the cause of widowed seniors cohabitating as some of them feel it is "shameful to remarry in their later years."

Li said it is "selfish and unreasonable" for younger generation to resent old people remarrying, as the elderly are entitled to a happy domestic life as anyone else.

Lawyer Wang Nailong at Liaoning-based law firm Longfeng said seniors' de facto relationships are unadvisable as such relationships are not legally protected in case of dispute over property and living expenses.

"When there is a dispute, they are no doubt worse off," he said.

Lawyer Fang Ting at Beijing-based law firm Yinke said in lawsuits related to seniors' remarriage, property issues are the most common point of dispute.

Fang, who has received many phone calls from old people seeking advice on remarriage, said old people should register for marriage to protect their own interests.

"Writing a will is also important. It ensures wishes are respected in terms of who receives their estate."