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Sorry, I wasn't clear. These pages would be on a sub-domain of their main site: landingpages.theirsite.com

But from Federico's answer that looks to make little difference.

We are working with a site that is difficult at best to update. The client intends to re-do the site in 18 months or so but needs to start generating more traffic (and sales) now. What are thoughts on creating landing pages as micro-sites that point to the current site conversion page as a stop gap? Beyond not sharing authority is there any known penalty? By the way they don't have tremendous ranking right now - often bottom of page two - and the micro-site won't duplicate any content.