In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: However , one trial analysed subgroups of 2839 relatively healthy 50 to 59 year - old women taking combined continuous HT and 1637 taking oestrogen - only HT , versus similar - sized placebo groups .
### Output Text:
However , one trial analysed subgroups of 2839 relatively healthy 50 to 59 year - old women taking combined continuous HT and 1637 taking <span class="chemical">oestrogen</span> - only HT , versus similar - sized placebo groups . 