Rachel DeWoskin (pictured), author of Foreign Babes of Beijing, wrote an article for The Times of London about mistresses or er nai in China. It's a few months old already but worth a read: Wife sentence. Excerpt:

At a dinner in Beijing this summer, my old friend Tang brought along a pale wisp of a girl in designer jeans and stiletto heels, whom he introduced as his girlfriend. None of the six of us already seated around a table in Beijing’s stylish Three Guizhou Men restaurant batted a single lash, even though Tang is married with two children – and we all know his wife. Bringing a mistress to dinner in China is not cause for comment, especially in a hip, expensive outfit like this, where patrons have to be rich to afford the food. Our table was covered with gorgeous dishes: translucent noodles draped with spring onions and hot peppers, spicy fish in a ginger broth and racks of tender ribs. Tang’s girlfriend reached her chopsticks out and a small piece of meat slipped off the bone.

I lived in China for six years in the 1990s, and was on a biannual visit there this summer. When I asked Tang’s girlfriend, “What do you do?” I knew it was too direct a question. But I asked it warmly and in Chinese, hoping not to embarrass but to include her. She didn’t appear to mind.

“She’s a TV hostess,” Tang responded, “very successful.” He put his arm around her, and she smiled in a way that suggested she was both smitten with and amused by Tang.

“Why bother working at all?” one of our male friends joked. “Tang can’t afford an apartment for you?”

“Wouldn’t that make you an ernai?” I asked, hoping for the “foreigner doesn’t get it” forgiveness exclusion. The men all looked at me, horrified...

On the subject of mistresses, if you suspect your husband has one and want evidence, you can try this Beijing-based investigation firm (Chinese only). If you are a mistress and want to know your rights and get other kinds of help, you can check out the Mistress Rights Protection Website.

Thanks to Will Freeman for the link.