StreamCorruptedException error, when deserealization of an object occurs

I've burned my neurons trying to fix this, if anyone can give a help; It follows the code that the error occurs. I'm trying to open subclasses that have been serialized, but this error does not know why.

try{
    fileInput = new FileInputStream("C:\Users\Trojers\Documents\NetBeansProjects\Trabalho\trabalho.txt");
    ObjectInputStream ler = new ObjectInputStream(fileInput);
    while(true){

    ArrayList<Aluno> al = (ArrayList)ler.readObject();  
    ler.close();
    System.out.print(al);

    }
} catch (FileNotFoundException ex) {
    System.out.printf("Nao encontrado");
    System.exit(0);
} catch (IOException ex) {
    ex.printStackTrace();
} catch (ClassNotFoundException ex) {
    System.out.printf("Nao encontrado.\n");
    System.exit(0);
}

StrackTrace:

java.io.StreamCorruptedException: invalid type code: AC
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1563)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:422)
at trabalho.Interface.imprimir_TodosAlunos(Interface.java:201)
at trabalho.Interface.mostrar_Dados(Interface.java:292) 
at trabalho.Interface.tela(Interface.java:54)
at trabalho.Trabalho.main(Trabalho.java:18)

The serialization of objects was done like this:

    //Inicializacao dos atributos 
    Aluno ob = new Pos_Graduacao_Aluno(nome, sobrenome, rga, nomeCurso, 
    ano_Ingresso, nome_Orient, sob_Nome, nota_Exame); //Aqui, instancio um 
    objeto do tipo Pos_Graduacao_Aluno, que extende de aluno  

    cadastrar(ob); //Metodo responsavel por colocar no ArrayList
    tela();
} catch(InputMismatchException ex){
    System.out.printf("Valor incorreto.\n");
    tela();
} 

public void cadastrar(Aluno e){ //Metodo que insere no ArrayList 
    try {
        objectOutput = new ObjectOutputStream(fileOutput);
        objectOutput.writeObject(e);
    } catch (IOException ex) {
        System.out.printf("Nao foi possivel salvar o arquivo.\n");
    }
}