XML serialization reference - duplicate

[Java or C#] I have some problem with serialization. How not to duplicate all the information about the object and use only reference?

Sample classes:

class Author {
  public String id;
  public String name;
}

class Book {
  public String id;
  public Author author;
  public String title;
}

And I have to format the output file like here:

<store>
  <authors>
   <author id="PK">
     <name>Philip Kindred</name>
    </author>
  </authors>

  <books>
    <book id="u1">
      <author>PK</author> <!--  use only ID -->
      <title>Ubik</title>
    </book>
  </books>
</store>